Add ui.footer-items to add a custom page footer

Summary: Fixes T6052. Allow installs to link to legal documents, etc., in the page footer.

Test Plan:

• Configured a footer.
• Viewed workboards (no footer).
• Viewed Conpherence (no apparent disruption, I think everything z-indexes over the footer).
• Viewed stuff on mobile (seems OK).
• Viewed login page (saw footer).

{F201718}

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T6052

Differential Revision: https://secure.phabricator.com/D10466