Prefill `@`-mention on thread replies

Problem to solve

When I speak to someone I make eye contact, and when I send someone an email I explicitly address them, and in GitLab the @mention is the closest equivalent.

In threads, people often reply without using an @mention which means I don't know it exists.

Further details

Proposal

Replying to a thread should prefill the input with an @mention to the previous participant to encourage their use.

Links / references