Enable configuration of an external Mongo server

[Sing-Li]

This must be done in such a way that:

1. by default, only the internal one is used - and the user should not have to explicitly specify anything for default behavior
2. if an external mongo is configured/specified and found to be working, then the internal one is disabled
3. cater for the external mongo to be located either on the same host, or over the network
4. assume the external mongo is already a primary in a replica set, but works even in the case where it is not part of a replicaset
5. works well with either internal caddy or external reverse proxy

WISH LIST:

1. Able to startup (from installation) with the external one enabled, and internal one disabled - via some pre-config.
2. Auto-migrate subcommand - including data export and re-import, from internal to external mongo