Refactor OmniAuth integration.

- Make use of Devise's OmniAuth integration.
- Merge config/initializers/omniauth.rb into config/initializers/devise.rb.
- Define button labels in config/locales/devise.en.yml.
- Fix devise's routes so we can use user_omniauth_authorize_path().

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It doesn't look great to have a global constant OMNIAUTH_PROVIDERS when Devise keeps a list of OmniAuth providers.
What do you think about refactoring our OAuth integration like this, @dsander, @cantino?