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

[mmd]

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

```ruby
  # Show a message
  def show
    @title = t ".title"
    @message = Message.find(params[:id])

    if @message.recipient == current_user || @message.sender == current_user
      @message.message_read = true if @message.recipient == current_user
      @message.save            <<<<<<<
    else
```

![image](https://user-images.githubusercontent.com/5842757/77845732-6ff99880-71b1-11ea-9444-94fbf034c9b3.png)


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