Prepare for 3.0.3 release

Created by: matthijskooijman

This PR starts adding some release notes for previously merged. This replaces #614, but this PR is created from a branch in this repository, making it a bit easier for all maintainers to update it as we go along.

As for release planning: I'd like to move to a 3.0.3 release, since we've already collected some relevant bugfixes in master. I would like to at least also merge #630 before releasing, which fixes the worst usability issues in the edit activity screen (but needs a bit more work, probably from my side).

Any other things that we would want to get into this release? I looked through the open PRs, but most of the recent ones still need more work, and the older ones need a better looking-at. #631 might be easy, but I haven't taken the time to review it yet (and I probably won't want to delay release for it).