Earlier, when a topic had less than 15 participants,
the @-topic typeahead was not visible. It should be visible
irrespective of the 'realm_wildcard_mention_policy' setting
when the participant count is not greater than 15.
The participant count for a topic can't always be calculated
accurately in the client as some of the messages might still
be loading.
We show @-topic in the typeahead whenever it might be possible
to use it.
We will give an error later if you aren't allowed to use it.
Fixes#27852.
This lets us simplify the long-ish ‘../../static/js’ paths, and will
remove the need for the ‘zrequire’ wrapper.
Signed-off-by: Anders Kaseorg <anders@zulip.com>