[EE] Epic comment thread

What does this MR do?

Adds comment thread to epic show page.

• NoteEntity splitted into multiple entities Project and Epic because of different paths/urls
• snippet-specific paths are removed from note entity because this code is not used anywhere
• both epic and epic notes are now awardable
• etag caching is enabled for epic notes (similar to issue notes)
• label references are not processed properly (not specific to comments) - follow up issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/5390, same for quick actions

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
  • Has been reviewed by Backend

What are the relevant issue numbers?

• #3889 (closed)