Show the customized "Login" message on the login screen

Summary: Depends on D19992. Ref T13222. If administrators provide a custom login message, show it on the login screen.

Test Plan:
{F6137930}

• Viewed login screen with and without a custom message.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13222

Differential Revision: https://secure.phabricator.com/D19994