Move Query class to core

Move the Query class from backdrop.read.query to backdrop.core.query.

The core of backdrop depends on this class so it should not be in one of
the app packages. I have left the functions used to parse a Query object
from a flask request in backdrop.read.query as they're specific to the
frontend.
Move response from read to core
self.self m8
Merge pull request #313 from alphagov/read2core

Move response and query to core [#72349958]