Quick action to add issue to epic with autocomplete

Follow up to https://gitlab.com/gitlab-org/gitlab-ee/issues/7287, but now you also have autocomplete, since https://gitlab.com/gitlab-org/gitlab-ee/issues/7472 is done.

That is, when you type /epic &, it will autocomplete to find epics that are in the immediate parent group of the project in which the issue is contained in.

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