Support notes without a project (personal snippets notes)

What does this MR do?

It prepares notes to be able to work without an association to a project. This is needed for creating notes for personal snippets which was requested in #15654 (closed)

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

I added a support for mentions for personal snippets notes. However I am not sure if other references should be supported - maybe links to snippets but not sure. Open for discussion.

Referencing users is quite core functionality so it needs to be doublechecked.

Why was this MR needed?

We need to support notes without project in order to implement a possibility to comment on personal snippets.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• 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?

Preparation for #15654 (closed)