config service: convert backing database to use mongodb

This is a much more natural fit for the config service which is really a
key value store, and has been abusing SQL. This will let us expand the
config stuff later to support profiles and per-account config values.

I have left in the SQL code for now so config changes are still being
sent there in the event of catastrophic mongodb failure.

http-service/src/main/resources/application.yaml

@@ -29,6 +29,9 @@ redis:
   pool.size: 10
   host: http://localhost:6379
 
+mongo:
+  host: mongodb://localhost:27017
+
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