[openstreetmap/openstreetmap-website] WIP: Convert database config to support reading from replicas (#2563)

[Andy Allan]

> Here's one candidate: reading a message (via GET /message/id) updates the message_read flag:

That's a perfect example! Thanks for finding it. So we have two things we need to do:

* Most importantly, figure out how to run the tests so that they use read-only connections when they are using the `reading` connection. If we don't do this, then there's a risk that the tests will pass and stuff will break in production.
* Figure out what to do on a case-by-case basis. I suspect if we just find a few methods like this (where the write to the database is a side effect of a larger purpose that's read only) then we can wrap the line or two in a block that uses the `writing` connection. 

If anyone has any suggestions on the first topic, I'm all ears. I've marked this WIP in the meantime.

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