Frontend : have issue/merge request title handled `code line`s, _italic_, **bold**

Summary

code lines, italic, bold are not handled in issue/merge request title now.

Why this matters and how we measure

1. Inconsistency (and: @mention, #476513 (closed) are handled!).
2. It is convenient to see code names/etc. highlighted. The same for the titles!
3. Everybody do this!

Proposal

Have issue/merge request title handled code lines, italic, bold.

Thanks!