This is a preparatory commit for using isort for sorting all of our
imports, merging changes to files where we can easily review the
changes as something we're happy with.
These are also files with relatively little active development, which
means we don't expect much merge conflict risk from these changes.
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.
Tim Abbott
rht
Steve Howell