WIP: Personal snippets comments

What does this MR do?

It adds a possibility to add notes to personal snippets.

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

This is a bit bigger change and multiple parts of system were touched. Please check especially

• adding notes for project snippets
• uploading files - not only for personal snippets but also for normal snippets / notes
• permissions - logged user who has access to a personal snippet can write notes (all users for public/internal snippets, only owner for private snippet), all users who have access to a snippet can read notes or download files
• Snippets::UploadsController - not sure if the inheritance from Project::UploadsController is correct here but I didn't want to duplicate code, concern might help here? (similar might be for other classes, will go through them again)

What is not included in this Merge Request (for discussion: include it here or create separated issues?)

• API support was not added
• notes discussion not supported
• todos not integrated, notes can't be resolved (should they be resolvable? if so additional changes will be needed)
• mentions not working yet - see discussion - should this be part of this MR?

Why was this MR needed?

Requested in #15654 (closed)

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?

Closes #15654 (closed)

What possible problems were found during the implementation

No issues created yet for these possible bugs:

• project/personal snippets - no possibility to delete attachment for notes
• project snippets - when a note is created and a user who does not have an access to the snippet is mentioned he/she receives the email anyway (and then sees 404 page)