Warn users when they are about to ping a group of users

Problem to solve

From @ClemMakesApps:

We really should add that feature that slack has, warn you that you are about to ping X number of people before allowing you to comment with that info

Target audience

Users of group aliases in comments and descriptions of Issues, MRs, etc.

Further details

It's a common occurrence within GitLab that users will accidentally ping a group that could contain numerous users (or even all users of an org).

We should display some warning to notify (or confirm?) that the user wishes to ping so many users.

Proposal

Display some kind of warning or confirmation request that the user wants to ping a group containing numerous users.

What does success look like, and how can we measure that?

Reduction in accidental pings to groups.

Links / references