This can be squashed with the prior commit or subsequent commit,
or it can just stand on its own, but it's part of transitioning
to a functional change in the next commit.
(imported from commit 155a0cdd28f851810fbedfef1a306e3190bf1c34)
1) The class Filter now lives in its own module.
2) The function canonicalized_operators() is now a class method on Filter.
3) The function message_in_home moved to filter.js and became private.
4) Various calling code had to change, of course.
5) Splitting out Filter helped simplify a few tests.
(imported from commit e41d792b46d3d6a30d3bd03db0419f129d0a2a7b)
