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).
Reviewers: btrahan, chad
Reviewed By: chad
Subscribers: epriestley
Maniphest Tasks: T6052
Differential Revision: https://secure.phabricator.com/D10466