If we have a stream named "Denmark" and we're narrowed to it,
then use "Denmark" as the default stream name in the compose box
even if the narrow operators are lowercase.
(imported from commit e9f06b7307c73231aa887dc95849e0307984e6f0)
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)
There are also one or two places we don't need to use it for security
purposes, but we do so for consistencey.
(imported from commit aa111f5a22a0e8597ec3cf8504adae66d5fb6768)
