Make user mentions case-insensitive

What does this MR do?

It makes rendering usernames case-insensitive.

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

Non-lating usernames won't be covered by this fix for Ruby < 2.4.
Since username's validations don't let to use non-latin letters, I'm not sure this would be a problem.

Why was this MR needed?

It makes UX simpler.

Screenshots (if relevant)

Before:

After:

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 #24493 (closed)