Allow users to "request full access" to the directory.

**NOTE:** The original title of this issue was "add support for membership applications" but that has changed.

As #31 mentions, users who haven't been vouched-for have as little access to the site as anonymous users. The move to a multi-city directory (see #27) further complicates matters: because there's no way to affiliate non-vouched users with a Hive city, staff have no idea how to make sense of them.

This PR adds support for membership applications, whereby users are guided through a basic application process, and relevant staff are notified of new applications.

## User Flow

When a user who isn't already vouched-for visits the directory's home page, the following alert will *always* appear at the top of the page:

![alert](https://cloud.githubusercontent.com/assets/124687/4201503/3e0da41e-381f-11e4-906b-4e79fea3bca6.png)

Upon clicking the link they will be taken to `/accounts/apply/`, which looks like this:

![apply](https://cloud.githubusercontent.com/assets/124687/4201507/5102817a-381f-11e4-9e64-4e25af1e4602.png)

Note that if the site is a multi-city install (i.e. the Hive Global Directory), the city field won't be auto-populated, and the user will be forced to choose from a list of all Hive cities.

All fields on this page are required. Once successfully submitted, the user will be redirected to the directory's home page with the following alert at top:

![success](https://cloud.githubusercontent.com/assets/124687/4201529/8b90faa6-381f-11e4-812b-57c2352ecc31.png)

At this point, the fact that the user has filled out the application form is recorded **in their user session**, so they will not be alerted to fill out the application form again unless they log out and log back in (or log in from another device, etc).

Meanwhile, an email will have been sent out to either all staff in the selected Hive city **or** all superusers (if no staff exists in the Hive city). The email will look something like this:

```
Subject: johndoe has applied to join Hive NYC!
From: no-reply@directory.hivenyc.org
To: julia@hivenyc.org

Hi!

John Doe has just applied to join Hive New York City.

Here is what the user had to say:

"""
Hi, I met Lainie at the Maker Party at Tuesday and she told me to sign up for the Hive!
"""

To vouch for this user, visit the following URL and associate them with
an organization:

https://directory.hivenyc.org/admin/auth/user/157/

You may also want to email the user at johndoe@example.org to let them
know that they've been vouched for.

- Your Friendly Hive Admin Robot
```

## Out Of Scope

* It's clear that blasting out an email to *every* staff member when a user submits an application probably won't scale very well. Odds are that Hive orgs might want to assign only one or two members of their staff to responding to applications. Right now, though, the blast is better than doing nothing at all, and we don't know for sure how much application volume there will be, so it's easiest to start out with this simple solution and adapt later if needed.

## Technical Considerations

I didn't want to have to bust a database migration, or deal with managing application models, so I just had the application form send out an email. If we really need to store the application in the DB, we can bust a migration for that later, but hopefully we won't need to.

## Things Left To Be Done

- [x] Document the UX on this PR and make sure stakeholders are okay with it.
- [x] SIngle-city installs shouldn't ask the user for their Hive city (or at least, the Hive city should be auto-populated).
- [x] Email domains with multiple organizations (e.g., Iridescent NYC and Iridescent Chicago) should never be auto-vouched to one of them arbitrarily.
- [x] Unit tests.