Handle error where Services team already exists

Error reference:
https://app.getsentry.com/app20126465herokucom/production/group/18233199
Don't check for line presence in the latest commit

This fixes an issue that Hound wound't comment when multiple commits got pushed
to a PR at the same time. As it was, Hound would check if only the head commit
included the line with violation -- resulting in no comment if the violation
came in the previous commit.

Instead, simply check if the violation on the line was previously unreported.
Add account page

* show repo totals
* show a list of billed repos
* refactor external customer data fetching to `PaymentGatewayCustomer`

https://trello.com/c/6AyMCv1t
Re-structure style guide configuration

`.hound.yml` is now used to enable/disable language support and provide the
location of the configuration file.

Example `.hound.yml`:
```yaml
ruby:
  enabled: true
  config_file: .rubocop.yml

coffee_script:
  enabled: true
  config_file: config/coffeelint.json
```

* introduce `RepoConfig` class to handle configuration fetching and parsing
* extract `StyleGuide::Base` for common style guide logic
Support legacy config
Document new configuration setup
Re-factor StyleGuide for ease of type addition

This change makes it easier to understand `StyleGuide::Base` inheritance. The
template method pattern makes it easy to scale style guides to scale
horizontally: just add the new language style guide and inherit off of `Base`.

Changes:
* add `Violations` object to manage unique violations
* refer to modified lines as changed to make it easier to reason about
* introduce `UnchangedLine` class
Merge remote-tracking branch 'upstream/master'

Conflicts:
	app/models/analytics.rb
	app/models/repo.rb
	app/models/style_checker.rb
	app/models/subscription.rb
	app/models/user.rb
	app/serializers/repo_serializer.rb
	app/services/repo_subscriber.rb
	app/views/application/configuration.haml
	app/views/shared/_header.haml
	app/views/templates/repo.haml
	spec/controllers/subscriptions_controller_spec.rb
	spec/factories.rb
	spec/models/style_checker_spec.rb
	spec/models/style_guide/ruby_spec.rb
	spec/services/repo_subscriber_spec.rb