Use more defaults to reduce necessary config

[ThrawnCA]

It would be possible to use more default settings in order to reduce the amount of configuration needed for this extension. For example:

• Redis settings could default to localhost, port 6379, db 1, or perhaps reuse ckan.redis.url with an incremented db number.
• CKANLoginThrottle and BeakerRedisAuth could be inserted by monkey-patching UsernamePasswordAuthenticator.
• A default value for ckanext.security.domain could be extracted from ckan.site_url since that should use the externally visible domain.

[markstuart]

These are good ideas @ThrawnCA, I'll see if we can get some budget to tidy this up a bit.