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>

* Handle push rejection message in Merge

* placate golangci-lint

* Fix sanitize, adjust message handling

* oops

* Oops

* Handle push-rejection in webeditor CRUD too

* Apply suggestions from code review

Co-authored-by: Lauris BH <lauris@nix.lv>

* Allow adding collaborators with (fullname)

Signed-off-by: Sasha Varlamov <sasha@sashavarlamov.com>

* Refactor username suffix to utils pkg

Signed-off-by: Sasha Varlamov <sasha@sashavarlamov.com>