Clean Issuables milestones so that records are valid
We added a validation that a milestone belong to the same project (under a group) as an Issuable milestone is set on. See https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/2788
However we now can have milestones that don't meet this validation and the issuables with them can't be updated anymore.
For issues and merge requests there should not be many cases (if any) - it was not possible using normal UI workflow, there had to be an intention. However it might have been possible using API. We should check all issues and MRs if they have a valid milestone and set to nil if not.
For epics we set a milestone when promoting an issue to an epic in https://gitlab.com/gitlab-org/gitlab-ee/blob/master/app/services/issuable/clone/attributes_rewriter.rb#L19. We, however, don't support milestones on epics so we should set milestones to nil. Before that gitlab-ee#10270 should be done.