Redmine

Hello, since nobody has responded to my comment/patch on the old ticket, I will give it another try with this new ticket and proper tracker type (tracker=Patch).
See also issue #16188.

Based on the outdated plugin I tried to integrate the changes into current git master, tested the UI with a running server and created some unit tests.

The plugin was running fine with redmine 3.x, but for redmine 4 the overwritten files had changed so the UX got broken and also the routes needed to be fixed (since this seems new with redmine 4.x).

I'm not 100% sure if the version in migration is right (6.1?) and if the config/routes.rb is properly since I'm not really into ruby/rails dev (so sorry if this is not proper coded). I mainly ported the plugin into the source. For me the code worked incl. routing and it provided same functionally than the old redmine plugin.
Tests were green beside one search test (Must be superuser to create this extension).

If the code/patch needs adaption, please let me know.