Adds required API and front-end changes to modify and read the
wildcard_mentions_notify field in the Subscription model.
It includes front-end code to add the setting to the user's "manage
streams" page. This setting will be greyed out when a stream is muted.
The PR also includes back-end code to add the setting the initial state of
a subscription.
New automated tests were added for the API, events system and front-end.
In manual testing, we checked that modifying the setting in the front end
persisted the change in the Subscription model. We noticed the notifications
were not behaving exactly as expected in manual testing; see
https://github.com/zulip/zulip/issues/13073#issuecomment-560263081 .
Tweaked by tabbott to fix real-time synchronization issues.
Fixes: #13429.
This change makes it possible for users to control the notification
settings for wildcard mentions as a separate control from PMs and
direct @-mentions.
Nat1405
Tim Abbott
Rishi Gupta