[EE] Add discussion API

What does this MR do?

• adds basic discussions API for issues, snippets and epics
• adds notes API endpoint to epics
• reorganizes notes specs (so same tests can be used for all noteable types - issues, MRs, snippets, epics)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug

What are the relevant issue numbers?

gitlab-ce#41860