Store assignees for merge requests in elasticsearch
The following discussion from !10161 (merged) should be addressed:
@oswaldo we also need to support multiple merge request assignees in the elasticsearch integration. Fortunately, this is quite easy to do!
Look at https://gitlab.com/gitlab-org/gitlab-ee/blob/master/ee/app/models/concerns/elastic/application_search.rb#L87 for a comment explaining how we did it in a (backward-compatible) manner for issue assignees.
It turns out that currently, we don't index assignee_ids for merge requests at all. This means that it's impossible for us to, say, implement filtered search against the elasticsearch backend.
We do store assignees for issues, and do so in a way that's compatible with multiple assignees, so this should be an easy addition.