Upgrade to Rails 6

I propose we upgrade ASAP to Rails 6. I was ambivalent about the features added until i saw this:

https://edgeguides.rubyonrails.org/active_record_multiple_databases.html

This will become more useful as we pursue a distributed, fault-tolerant infrastructure and serve up read-only data via an API.

One database connection can be to our primary, read-write node, while another (powering the API) can pull form our replica, read-only PG nodes.