Add hook EnhancedChangesList::getLogText

Add hook EnhancedChangesList::getLogText

Enhanced RC generates these "(3 changes | history)" links for
every block of grouped recentchanges. That changes-link links
to a diff page.

For Flow, that is all wrong: we have different ids (not integers),
on a different page (&curid=&oldid=&diff= means nothing). Even
the concept of a "diff" page seems wrong here for us - a new post
is not part of some document that can be diffed.
In short: we'll want to generate a different link, and we'll need
a hook to let us change them.

Meanwhile also split the code that generates those links into a
separate method.

Bug: T72513
Change-Id: Ib32fb9552b80f9581d89b3b47da6e5d32e3d84a3