Use vanilla redis library for User persistence

Before this change, the redis-backed Rails::Cache from the `redis-rails` gem
was used to persist the user properties across nodes. This was problematic because:

* the gem used object marshalling which ocasionally started failing (eg when the ruby version was upgraded)
* the `redis-rails` gem appears to not be maintained anymore

The new approach is to use the `redis` gem directly and to serialise the user properties as JSON.

/cc @alext would be really good to get your opinion on this.