Add dedicated Stack Overflow jump page

## Introduction

There are many [spring-related tags](http://stackoverflow.com/tags) tags on Stack Overflow today. This new page on [spring.io](http://spring.io) will:
- clearly communicate to users that Stack Overflow is the place to go for Spring Q&A
- help provide guidance about which tags to use for what, i.e. which tags the Spring team is actually monitoring
- provide a convenient stream of all the latest Spring-related questions (something that's not easy to do unless you configure filters, etc on SO. _UPDATE: actually, it's [easier than we thought](http://stackoverflow.com/questions/tagged/spring-*), but most folks probably don't know you can do that._

## Basic requirements

This new page should have:
- a link in the site-wide top nav that reads Q&A, pointing to new page at http://spring.io/questions
- at the top of page (in full width):
  - prose explaining that users can ask and answer Spring-related questions at SO, etc
- on the right 1/3 of the page:
  - each Spring project, with its icon and link to dedicated project page
  - under which is a list of monitored Stack Overflow tags for that project (hard-coded)
  - each tag links to it's respective location at SO, e.g. [spring-boot](http://stackoverflow.com/questions/tagged/spring-boot)
  - tags mirror SO style/css
- on the right 2/3 of page:
  - use the [SO API](http://api.stackexchange.com/docs/) to list latest questions across *all* monitored spring-* tags. (Re API, see also [this](http://api.stackexchange.com/docs/questions#order=desc&sort=activity&tagged=spring-data&filter=default&site=stackoverflow&run=true) and [this](http://meta.stackoverflow.com/questions/115927/can-i-embed-a-stack-overflow-tag-page-on-my-website) and [this](http://stackapps.com/))
  - each question title is a link to the actual question at SO


----

## v1 (MVP)

### Goals

  1. Demonstrate to users that there is a great place to engage in Q&A around Spring
  2. Show that there's plenty of activity there
  3. Educate users about which tags the Spring team is following, and which tags we consider to be associated with each Spring project.
  4. Call users to action to ask questions.

### Comp

Thanks to @sdunn, here is the comp that what we're working from (as of May 6th):

![bc0ac6fe-d504-11e3-98be-29daaa10fab8](https://cloud.githubusercontent.com/assets/301810/2899468/032e6434-d5b7-11e3-83c2-4c995dccc216.jpeg)

### Differences in v1 from the comp above:

 - [ ] No blurb at top. Instead an "ask a question" button. Maybe one sentence of context.
 - [ ] Somewhere on the site, mention that SO *is* the new forum, and go here to see the old forums.
 - [ ] Tags on right-hand side are each links, e.g. [spring-security].
 - [ ] In the left column, show all tags associated with a given question, but color code the "spring tags" with the same color we use on the right-hand column.
 - [ ] The questions showing up in the left column are "active" questions, like those found here.
 - [ ] Remove the "Login or Sign up" text / bar from top.
 - [ ] NO responsiveness in this version.. i.e. shut it off explicitly, but DO ask Shannon about his thoughts when we hand it off to him for styling.
 - [ ] Instead of "Frequent Questions from StackOverflow", make the title (something like) "Recent activity across all Spring tags on StackOverflow", and turn that text into a link representing the [search query we actually used](https://api.stackexchange.com/docs/search#page=1&pagesize=10&order=desc&sort=activity&tagged=spring-mvc%3Bspring-data%3Bspring-security&filter=default&site=stackoverflow&run=true) to get those results.

### Implementation plan

_All of the following to be committed against the 'stackoverflow' branch for this PR._

 - [x] Basic infrastructure: controller, template, tests
 - [x] Add stackoverflow tags to projectmetadata schema (+ add'l table if necessary)
 - [x] Prepopulate tags for a few projects (for ease of use when developing locally)
 - [x] Render SO tags per project on the right 1/3
 - [x] Make each of them a link
 - [x] Pull in one page (10? 20?) of latest "active" questions, and display all data points as shown in the comp. Again, Do not worry about styling at this point.
 - [x] Hand off to @sdunn for styling. Let him know << ask his thoughts on responsiveness
 - [x] Allow for populating tags in /admin/projects
 - [x] Double-check rate limiting, caching, etc
 - [x] Change title to "Spring on StackOverflow"
 - [x] Delete second-level title

----

_The following are ideas about things we can do beyond v1. New issues should be created as appropriate._

## v1.1

1. Update project pages to show all associated SO tags, not just a single SO icon. This will require the addition of a new section on the right 1/3 of the project pages, update to shared gh-pages templates, etc.

## v2

On the /team/{user} page:

1. Add support for entering stackoverflow username
2. add widget to list recent activity (like we do for twitter)

## v3.

On the /questions page:

1. Show recently active users against all tags / specific tags
2. Highlight which among those users are Spring Team members

----

## Meta-tasks

 - [x] @bclozel and @cbeams to review comp and refine requirements (May 6)
 - [x] @cbeams to transcribe notes from the above into this issue (May 7)
 - [x] @bclozel and @cbeams to take a first pass at implementation (May 7)
 - [x] @bclozel to integrate @sdunn work
 - [x] @bclozel to polish design integration after feedback (should we remove the "Spring StackOverflow title? Where should we put the ask a new question button? the SO logo?)
 - [x] update the menu link (change "forum" to "questions"?)
 - [x] @bclozel to finish v1 implementation