EPIC: Support a secondary database

[obscurerichard]

Story

As a Freezing Saddles administrator, I want to be able to switch between two databases, in order to support database migration between multiple database servers.

Implementation Details

It would be great to have a secondary database source available to freezing-web and freezing-sync so that it would be possible to scale down from the in-season configuration with MySQL Aurora RDS to a built-in MySQL Docker container database server. That would save probably $800 per year in AWS costs.

If the app knew about 2 database sources, and could switch live between them, pausing syncs and new logins/registrations while the transition was underway, this could even be done live during the season.

The extra small version of this would rely on a manual backup / restore procedure and pausing the sync process and maybe configuring the site to prevent logins for a time while maintenance is underway.

The extra large version of this would have database backup and restore included, and be coordinated via messaging between the system components and shared state, either filesystem or an in-memory cache (why don't we have that already?)