Allow installs to configure an arbitrary block of HTML to show on the login screen

Summary: For secure.phabricator.com, I've wanted to put up a "you can use demo/demo" message for a while. Wikimedia also wants to add some custom login instructions.

Test Plan:
Configured this:

'auth.login-message' =>
  '<div style="width: 50%; margin: 1em auto; padding: 1em; border: 1px solid green; background: #dfffdf;">'.
    '<strong>Welcome!</strong> Lorem ipsum...'.
  '</div>',

...got this:

{F17167}

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran, jeremyb

Maniphest Tasks: T1637

Differential Revision: https://secure.phabricator.com/D3288