Allow configuration of Redis DB and cache DB
Allow configuration of Redis DB and cache DB

Hardcoding the Redis DB and Redis Caching DB to 0 and 2 in
`config/database.yml` makes an unsafe assumption that Discourse is the
only application using that install of redis-server. Instead of forcing
users to undergo yet another form of configuration, allow Discourse
admins a nicer way to configure the Redis databases used.

Signed-off-by: David Celis <b1c1d8736f20db3fb6c1c66bb1455ed43909f0d8@davidcel.is>

tadp/learnswift,tadp/learnswift,tadp/learnswift