[openstreetmap/openstreetmap-website] Upgrade to Rails 6 (#2352)

[Andy Allan]

Rails 6.0 [has been released](https://weblog.rubyonrails.org/2019/8/15/Rails-6-0-final-release/), and it contains some things that will be of interest to us:

* Parallel testing - we have a lot of tests!
* Multiple databases - this will allow us to split database read requests into their own connection, and route them to the database replicas. This is something cgimap already does and it will be great to have this here too.
* Action Mailbox - this would be a lower priority, but it will allow us to use standard methods for processing incoming emails. It would be of most use to secondary deployments, who might have less control over receiving their emails than OSMF does.

The major blocker for the upgrade is, as ever, the [`composite_primary_keys`](https://github.com/composite-primary-keys/composite_primary_keys) gem, which needs a 12.x release to support rails 6. We can wait on that, for now.

I've looked through the rest of the [release notes](https://edgeguides.rubyonrails.org/6_0_release_notes.html) and the [upgrade guide](https://edgeguides.rubyonrails.org/upgrading_ruby_on_rails.html#upgrading-from-rails-5-2-to-rails-6-0), and nothing particularly sticks out to me. If there's anything else that will block our upgrade, or any other new feature that we should also take advantage of, add your comment here!

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