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 index is basically free, since it only consumes space for
messages that mention the user, but results in a massive performance
improvement when querying messages that mention a user.
Tim Abbott
Anders Kaseorg
rht
Steve Howell