return true if milestone is not available for issuable
What does this MR do?
We introduced validation for milestones as the part of security release. We now check if milestone belongs to the same project/group as an issue/MR. We don't support milestones for epics.
But in AttributesRewriter we copy milestone to Epic. That won't work now because of that new validation. Additionally epics that have milestone copied can't be edited anymore.
This MR fixes that by returning true in validating function in case an Issuable is an Epic (discussed in https://gitlab.com/gitlab-org/gitlab-ee/issues/10201#note_147272331).
We will need an additional migration in order to clean the data.
This change has been already approved on dev (https://dev.gitlab.org/gitlab/gitlab-ee/merge_requests/857#note_155755) but was lost due to conflict resolution when merging on gitlab.com
What are the relevant issue numbers?
https://gitlab.com/gitlab-org/gitlab-ee/issues/10201