CSS: Fix vertical position of page footer
Summary:
rPd3cbfd95 introduced a regression on some pages when a page footer is configured via `/config/edit/ui.footer-items/`: The footer is not displayed at the bottom.
Closes T16147
Test Plan:
* Configure a page footer via `/config/edit/ui.footer-items/`
* Visit a lot of pages (incl https://we.phorge.it/uiexample/view/PHUIFeedStoryExample/ on which I found this problem) and scroll down (and be aware that some pages like project workboards do not display a footer anyway).
Reviewers: O1 Blessed Committers, amybones, mainframe98
Reviewed By: O1 Blessed Committers, amybones, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16147
Differential Revision: https://we.phorge.it/D26133