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:
Screenshot_20170328_223501

After:
Screenshot_20170328_223430

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

Closes #24493 (closed)