Refactor config-passing throughout, no longer passing Config object.

Previously, `Config` objects were passed around, with `Listener`s
extracting config needed. Instead, be explicit about config passed.
Add `App#check_all_imap`,`#check_all_ftp`, dispatching from
`#refresh_loop`.
Install redis-namespace, simplifying Redis namespaces throughout.

Wrap `App#configure`,`Notifier::Sidekiq#connect_redis` Redis
connections using `Redis::Namespace`. Remove `:redis_ns_ryespy` passing
in `App#check_all_imap`,`#check_all_ftp`. Rename `Notifier::Sidekiq`
`:redis_ns_notifier` => `:namespace`. Update to remove trailing `:` in
namespaces throughout, as redis-namespace adds this automatically.

Note that this changes namespacing behaviour slightly, in that now
setting `--redis-ns-ryespy`,`--redis-ns-notifiers` will result in a
`:` being added to the namespace (whereas previously it was necessary
to include this as part of the namespace).