Merge branch 'js-tooltips-for-mentions' into 'master'

Use JavaScript tooltips for mentions

## What does this MR do?

- use JavaScript tooltips instead of plain title for mentions (issue, merge request, snippet, user, group, commit, commit range, milestone)

## Why was this MR needed?

JavaScript tooltips look nicer and can wrap.

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5261#note_13106713, fixes #21500

See merge request !5301