Do not record 4xx class as error

User errors should not be logged as application errors. Doing so masks
real errors.
Attach logging to root handler

This means that we will capture log messages that are not specifically
written to flasks logger.

Before this commit we're not capturing any error logs from the rest of
our application or from dependencies (unless they cause exceptions that
bubble up to the flask layer).
Merge pull request #340 from alphagov/backdrop-errors

Do not record 4xx class as error
Merge pull request #341 from alphagov/attach-logging-to-root-handler

Attach logging to root handler
Merge pull request #342 from alphagov/subgroup_keys

remove_keys() takes a list of keys, not one key