This commit completely switches us over to using a
dedicated model called MutedTopic to track which topics
a user has muted.
This includes the necessary migrations to create the
table and populate it from legacy data in UserProfile.
A subsequent commit will actually remove the old field
in UserProfile.
This makes get_stream match get_realm, get_user_profile_by_email,
etc., in interface, and is more convenient for mypy annotations
because `get_stream` now doesn't return an Optional[Stream].
The goal of this library is to make it a lot easier to prevent bugs
like CVE-2017-0881 by having all of our views logic for fetching a
stream go through a couple carefully tested code paths.