Improve server side caching

[jcraigk]

We're using memory_store for caching API responses which is fine for single server instances. It gets cleared every deploy though. It would be nice to use memcached, but the current production Dokku deploy is throwing permission errors when trying to create an instance. Same with a new redis instance. The server has been up for years without restart and I'm nervous to restart it now so to fix this we'll likely want to migrate to a fresh server.

https://github.com/rails/solid_cache

Related to #391