2.15.0 release candidate

Replaces !574 (closed). Fixes #449 (closed).

This includes my own !572 (merged) and !571 (merged), Ico's !554 (merged), !557 (merged), !563 (merged), !567 (merged), and !568 (merged), and last but not least Guillem's !560 (merged). All fully merged, with merge conflicts fixed and with proper merge commits included.

These are all finished, have been tested very extensively, and are as ready as they'll ever be without getting them out in a release.

All that remains to be done is merge this MR to master and tag the release.

NOTE: Once this is merged, !572 (merged) and !571 (merged), !554 (merged), !557 (merged), !563 (merged), !567 (merged), !568 (merged), and !560 (merged) will all effectively be merged as well and so can be closed (unless Gitlab discovers that on its own; Github is clever enough to do this, but I'm not sure about Gitlab).