Remove references to Redmine in code

See #4400. This should be all of them, except for, of course, the module that targets Redmine itself.

Note that this also updates the README.md with more current information as well.

We should not expect our Redmine instance to be up for much longer. If the bugs that say "See Remdine issue 1234" are not actually resolved yet, my guess is that those issues should be recreated on GitHub. However, if they haven't the bugs described are either resolved, or haven't been important enough to survive triage.