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.
Missed one in CONTRIBUTING.md
Normalize links to metakitty, unleashed, and wiki
Land #4439, rm Redmine references in code
Merge remote-tracking branch 'upstream/master'