docs: added changelog for 4.12 release.

diff-cache: use bz2 to reduce diff-cache size.

diff-cache: remove diff cache dir on repository removal.

diff-cache: show safe relative path where cached diffs are stored.

diff-caches: show count and size in caches view per repository.

api: added audit logs for user-group related calls. - fixes #5461

caching: add option to cache diffs for commits and pull requests. - new option for vcs settings to enable caching of diffs and commits - enables auditing of PR after closing it, and removing commits from source repo - speed up for large PR upto 20x - reduced memory usage

release: merge back stable branch into default

api: fixed problems with repository fork/create using celery backend. - fixes #5462

diffs: don't use recurred diffset attachment in diffs. This makes this structure much harder to garbage collect since it was before infinitly recursed.