We can un-clutter our message context menu, which we had to share
with user actions. (Even though we only had one so far.) I added one new
user-specific action which allows you to quickly mention the user in
chat. Otherwise you would've had to copy their username or use the
completion menu.
It's convergent on mobile, it still has the hover indicator and it also
is available through the AuthorComponent.
BUG: 486252
