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>.
Check repos for valid information when activating

* Don't activate repos where either `private` or `in_organization` is nil
* Return `true` when pushing to a job queue

Minor:
* 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
hound appeasement
more quotes
Remove true response
Revert actual attribute checking

* Remove `nil` checking for attribute in repo
* Remove specs
Move the initialization of the object into setup
omg hound, with the quotes already