Remove OAuthCredentials, Repository.credentials, Gitlab providers and dependency

closes #1679 (closed)

Removes :

• modules arkindex.process.providers and arkindex.users.providers
• in models:
  • User.credentials
  • Repository.credentials
  • Repository.hook_token
  • enabled, provider and provider_class methods on Repository model
  • OAuthCredentials
  • OAuthStatus
• in serializers:
  • git_clone_url (+ get_git_clone_url method) and enabled fields on Repository serializer
  • OAuth serializers
  • ExternalRepositorySerializer
• commands update_repositories_hooks and update_repositories_refs
• API:
  • GitRepositoryImportHook
  • ListExternalRepositories
  • CreateExternalRepositories
  • ListOAuthProviders
  • ListOAuthCredentials
  • Retrieve/DestroyOAuthCredentials
  • StartOAuthSignIn
  • RetryOAuthCredentials
• Gitlab OAuth check
• Gitlab / OAuth related settings