WIP: Resolve "Add epics to global search"

What does this MR do?

Adds epics to global search

TODO

• Finish elasticsearch
• Add to the search view
• Tests!

Are there points in the code the reviewer needs to double check?

• How epics are loaded in Gitlab::SearchResults
  • Currently loading all epics in all groups user is a member of
• ElasticSearch permission issues?

Screenshots (if relevant)

TODO

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• EE specific content should be in the top level /ee folder
• Conform by the merge request performance guides

What are the relevant issue numbers?

Closes #4745 (closed)