Provide helper links on file description pages.

These come in a few different flavours: no translations,
do you wish to start?; translations, just view; and
translations, view and translate.

The &chooselanguage=1 parameter doesn't do anything yet,
but it will shortly.

Change-Id: I360f2fdc219d96e5c10670c34796d6f40fbed2b4
Merge "Provide helper links on file description pages."