Remove of deprecated rspec-mocks methods

The "stub" syntax in rspec-mocks was replaced with "allow" syntax in
RSpec 3.
Eliminate warnings from using GitHub's preview API

If you don't explicitly add the `accept` option, Octokit prints a
warning to STDOUT every time you use part of the preview API.

There are no remaining warnings from GitHub after this change.
Use default show_exceptions value of false

Enabling this hides exceptions during integration tests behind an HTML
500 page, which makes debugging much harder. It can also hide errors, as
the test can render a 500 and still pass.

I didn't have redis running locally, which caused one test to fail. With
the previous setting, it took me 15 minutes to find the failure. With
the new setting, it immediately prints an error about redis.
Introduce a single subscription plan

https://trello.com/c/Cl6H6nIU/378-update-pricing

* Simplify pricing by having only one plan
* Reduce the price for organization repos
Update account page to support deprecated prices

https://trello.com/c/Cl6H6nIU/378-update-pricing
Add repo_id to Stripe metadata

* Combine `RepoSubscriber` specs
Save email address when creating new users

https://trello.com/c/ipauXs4L/384-save-email-when-new-user-is-created

* Remove EmailAddressJob and enqueue hook
Update landing page

https://trello.com/c/VfhDZEYm/334-add-pricing-somewhere

* Add pricing
* Reword Pull Request heading
* Allow users to view landing page without signing out
* Remove redirect to repo list when users are authenticated