Move Apple Touch icons to docroots

This patch reverts patch Ib5554c54729 for most icons; since
older versions of iOS will hit docroot/apple-touch-icon.png
by default whatever our settings are, and since newer versions
look in docroot when no icon is specified in the HTML <link />
tag anyway, let's just simplify things.

I'm putting all existing icons in their respective docroots,
and not defining any icons for wikis that don't have them yet.
The enwiktionary icon — being different from the default icon
for wiktionary.org wikis — will be loaded from bits.wm.org.
Should such a need arise, other icons could be loaded that way,
too.

The usabilitywiki and mediawikiwiki icons are moved to their
proper names (having been loaded via <link /> until now).

Bug: 27911
Change-Id: I2f0990fa087a7b2969c982d05187331e7005b64a