Move footer icons to assets/ directory

poweredby_mediawiki_88x31.png is straightforward, just need to update
some paths.

The six license icons are more problematic, as the paths to them are saved
in users' LocalSettings. We're remapping them in Setup.php.

Bug: 69277
Change-Id: Ic7c32e56043cfbf94ef2271de4ff41ef18fbeee7
Merge "Move footer icons to assets/ directory"