Rename "activity" report to "north star" report

Use "North Star" term to match the thoughtbot growth team's terminology
for [the only metric that really matters][only].

[only]: https://medium.com/@joshelman/the-only-metric-that-matters-ab24a585b5ea

> How many people are really using your product?

Identify the interaction that represents the moment users really get value from
your product and identify the threshold for how often that interaction is
appropriate within the context of the product.

For example, Hound's "north star metric" is:

> Repos with builds per week

When Hound reviews a pull request or a new commit to a pull request, it
"builds." This is the "aha moment" when a user is getting value from the
product. We care about this number because most teams relax on the weekends.
Use named pages for Segment, add page titles

* In order to track conversion rate from home page to sign up,
  we need to give the `page()` method a name.
  https://segment.com/docs/integrations/mixpanel/#page
* By re-using `page_title`,
  we can also set up a nicer sitemap
  and improve SEO.
Require `active_support` in fast_spec_helper

This prevents the following error when running the tests.

```
.../activesupport-4.1.5/lib/active_support/number_helper.rb:3:in `<module:NumberHelper>': uninitialized constant ActiveSupport::Autoload (NameError)
   ...
   from .../activesupport-4.1.5/lib/active_support/core_ext.rb:1:in `<top (required)>'
   from .../spec/fast_spec_helper.rb:6:in `require'
```

See also: <https://github.com/rails/rails/issues/14664>.
Refactor RepoActivator

* Move enqueueing of `OrgInvitationJob` into the activator
* Use instance method for activator to cut down on redundant argument-passing
* Sort private methods into order in which they are called
* Use named parameters
* Use `.fetch` for retrieving hash values (raises rather than `nil`s)
* Avoid ternary operators

https://trello.com/c/fABEW1eM/391-don-t-allow-repo-activation-if-repo-is-missing-privacy-or-organization-information
Default to hound's token for the API client

Most of the action happens via HoundCi's github token. Defaulting to
Hound's token makes this process simpler, and makes it easier to see
which actions require the signed-in user's token.
Add More FAQs

https://trello.com/c/xXsDq7fl/410-add-more-faqs
Upgrade to Ruby 2.1.4

https://www.ruby-lang.org/en/news/2014/10/27/ruby-2-1-4-released/
Add links to the config and faq page in README.md

In an effort to work on issue #480

https://trello.com/c/12v0OLgm/411-add-link-to-faq-and-config-pages-in-readme-file
Fix <li> style error

https://trello.com/c/QnaR3CYv/412-fix-style-error-in-ul://trello.com/c/QnaR3CYv/412-fix-style-error-in-uls
Upgrade to Ruby 2.1.5

https://www.ruby-lang.org/en/news/2014/11/13/ruby-2-1-5-is-released/
Add test for multiline argument rule