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