Backport of Multiple Assignees feature
What does this MR do?
This is a backport of Multiple issue assignees feature from GitLab EE
Are there points in the code the reviewer needs to double check?
On the CE side we are limiting the number of assignees by one.
Why was this MR needed?
To not have conflict hell
Screenshots (if relevant)
Does this MR meet the acceptance criteria?
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Changelog entry added, if necessary -
Documentation created/updated -
API support added - Tests
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Added for this feature/bug -
All builds are passing
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Conform by the merge request performance guides -
Conform by the style guides -
Branch has no merge conflicts with master(if it does - rebase it please) -
Squashed related commits together