Use Rails::cache for email login token

- Store token in Rails::cache instead of database
- Store remember_me and return_to in the token object
- Don't put extra params in the login link
- Use cache expiry to expire auth token
- TODO: make the token login a Warden strategy instead of controller filter
- TODO: remove email_authenticatable mixin and database fields
Moved token auth logic to a Warden strategy

Removed token auth logic from SessionsController and created an `:email_authenticatable` Warden strategy that contains calls to the tokens and setting cookies and redirect paths.
Remove token authentication columns from user

Since we are using `Rails::cache` to store the email authentication tokens, these are no longer being used.
Merge remote-tracking branch 'origin/devel' into feature/email_tokens_in_rails_cache

Conflicts:
	app/controllers/application_controller.rb
Remove old mailer templates
Merge remote-tracking branch 'origin/devel' into feature/email_tokens_in_rails_cache

Conflicts:
	Gemfile
	db/schema.rb
Add test for token invalidation on generation

Make sure that token does not get invalidated when new token is created for that user.
Remove ability to set token digest format
Remove unnecessary :js => true
Add more token auth specs

- specs for warden strategy
- checks that both tokens are valid when 2 are created
Merge branch 'devel' into feature/email_tokens_in_rails_cache
Merge branch 'devel' into feature/email_tokens_in_rails_cache

Conflicts:
	spec/requests/apis_spec.rb
Merge pull request #172 from 18F/feature/email_tokens_in_rails_cache

Use Rails::cache for email login token