Replace all calls to os.Remove/os.RemoveAll by retrying util.Remove/util.RemoveAll and remove circular dependencies from util.

Fix #12339

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: silverwind <me@silverwind.io>

* Add logging to long migrations

Also fix v136 to not use models

Signed-off-by: Andrew Thornton <art27@cantab.net>

* Ignore empty avatars

* fixup! Ignore empty avatars

Migrate avatar names to include user ID and the md5 hash.