This function will make it easier to unit test upcoming
changes related to stream counts.
This was mostly moving code, but one change is that we
don't call create_subs() in subs.js any more (which would
have been kind of circular dependency), since the only thing
that it did besides calling a more appropriate function
in stream_data.js was to generate a trigger that was
subsequently ignored and possibly a UI trap, as we don't
want to be messing with the stream sidebar when we go into
the stream settings page.
We now simply call exports.create_sub_from_server_data() for
newly encountered unsubscribed streams (which don't belong in
the sidebar anyway.)
This function used to live in subs.js. It's mostly a code move,
but I simplified the logic to determine whether it's subscribed
not to do a lookup into the same data structure that the sub
already came from.
I also added some tests.
