Don't depend on hacky $wgExtDistListFile file

Fetch the list of extension repositories from Gerrit itself,
instead of relying on a list of extension names.

Improve some i18n messages while we're at it too.

Change-Id: I3967cc558138938c9280a1844b80dd5e01a29cf8
Merge "Don't depend on hacky $wgExtDistListFile file"