dune-pdelab release 2.5

The Dune core 2.5 release is out for about a month. It seems time to think about the PDELab release, esp. w.r.t. the upcoming course in Heidelberg in March! I am hereby volunteering as a release manager.

As a first step, I would like to evaluate what needs to be done. For that purpose, I would like to ask @all to look through existing issues and merge requests they contributed to see what needs to be done to finalize things. I will start going through them as well, tagging them either 2.5 or 3.0. Please feel free to contradict if you do not share that opinion.

Please use this issue for general discussion on the release.