Attach files (including images) to epic description

What does this MR do?

It adds support for attaching files to the epics (group context)

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

Is it ok from geo point of view?

This needs a CE backport - should we include the new group path/controller there? I think we should as it is not epic specific

Why was this MR needed?

We want to add files upload support to epics

Screenshots (if relevant)

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 UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Closes #3938 (closed)