The import code wasn't setting subscriber_count at all, resulting in a
value of 0 when dealing with imports from 3rd party apps.
We run this unconditionally, also for imports from Zulip, since this
ensures we won't inherit incorrect values, if the data we're import has
them - e.g. due to some bugs that affected the server the data came from.
(cherry picked from commit 6ab30fcced)