Add `#without_feature` helper
Move feature helpers to own module

means that they can be included separately in mailers
Obey token_auth feature flag in inadequate_profile email

We generate the appropriate URL in the ReminderMailer, depending on the
feature flag.
Add test coverage for information_request mailer
Obey token_auth feature for information_request email
Disable token auth functionality if flag disabled

attempts to use token auth when disabled will result in redirection to
the login screen with a flash message saying that it has been disabled.

The token auth input box is removed from the login page.
Rubocop style
User update emails obey token_auth feature flag

If token_auth is disabled, emails will be sent with the full person url
instead of the url containing the token auth.

Also added unit tests for the UserUpdateMailer to test this behaviour.
Add documentation about disable_token_auth config setting
Merge pull request #11 from ministryofjustice/token-auth-feature-flag

Token auth feature flag