[openstreetmap/openstreetmap-website] WIP: Use database replicas for read requests (#3201)

[Andy Allan]

I've rebased this PR and resolved the conflicts today.

The fix that we need in Rails hasn't yet made it into a formal release, but I expect that there to be one (rails 7.1) before the end of 2022. I've [tested the code](https://github.com/gravitystorm/openstreetmap-website/commits/multiple_databases_4_rails_main) with rails `main` and it now works! (Glossing over some unrelated rails-7.1 related breakages).

So we're back to waiting, but at least we know that we can use our read replicas soon. The steps from here are:
* Wait for the next Rails release
* Update this app to work on that release (e.g. wait for composite_primary_keys and [activerecord-import](https://github.com/zdennis/activerecord-import/issues/787) to be updated)
* Rebase this PR
* Should be good to go!

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