Fix metadata footer positioning in fullscreen mode

Add position:fixed to the metadata panel to stick it to the bottom
of the page.

This reverts the change in Ic37b4150288055c3fae8d22919ed7b1249db1f09
I can't recall if there was some intent behind that or it was an
accidental deletion; but this seems to work fine in Chrome and
FF.

Change-Id: Ic5fdce5fb3d6884df782f8bc9ec5c9a5c67edbd6
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/805