Earlier, a 'large @-mention notification' warning that pops up
for stream wildcard mentions was shown for topic wildcard mentions
too, which is incorrect.
This commit fixes the incorrect behavior. We no longer show the
banner for @-topic mentions.
We don't need a banner for @-topic mentions, as those are much less
likely to be used without thinking about it and would rarely be spammy
for a lot of people.
Fixes#27767.
Prakhar Pratyush