This sets us up to use better system-wide data structures
for tracking subscribers.
Basically, instead of storing subscriber data on the
"sub" objects in stream_data.js, we instead have a
parallel data structure called stream_subscribers.
We also have stream_create, stream_edit, and friends
use helper functions rather than accessing
sub.subscribers directly.
We rename all_everyone_warn_threshold to
wildcard_mention_large_stream_threshold as we would
be adding wildcard_mention_policy and this
constant will also be used to show error
in case when wildcard_mention_policy is set
to admins only.
Also add helper functions needed.
`select_item_via_typeahead` has been ported from casper
and is exactly same in puppeteer to as I couldn't find
any better way for that purpose.
Anders Kaseorg
Steve Howell
sahil839
Dinesh