[Completes #68827530] Return to App link on Sign In Page
[Completes #68884384] User views beta message in a popup view

Tweaks to Beta message and to message placement
Merge branch 'add_return_link_for_apps' into beta_warning_message

Conflicts:
	app/views/users/sessions/new.html.haml
	config/locales/en.yml
	spec/requests/oauth_apps_spec.rb
	spec/requests/users_spec.rb