[Newsletter subscriptions]

  - Add new subscriptions#create endpoint in order to handle
    new subscriptions.
  - Refactor NewletterSubscriber model.
  - Refactor newsletter form views.
  - Remove obsolete files.
  - Remove unneeded specs.
  - Full spec coverage.
Merge pull request #215 from insignia4u/newsletter-subscriptions

[Newsletter subscriptions] Big refactoring