Add a read-only mode

[jonathansick]

Add a read-only mode that can be enabled though an admin-only API route (and backed by the SQL DB).

Add endpoints that change DB state (POST, PATCH, DELETE) will check this DB field and return an error if it is set.

This feature will make DB migrations easier by allowing a second DB to be created and copied from the original, have the migration applied, and then allow the app to be deployed against the new DB.