Support comments for personal snippets

What does this MR do?

This is the last MR that puts all previous changes together and finally adds support for posting and displaying comments for personal snippets. These comments can contain files and a users can start discussions. Notes are not resolvable (same as for project snippets)

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

Why was this MR needed?

With comments personal snippets will be more usable.

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
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #12910 (closed)