Migrate to SQLite

[Wingysam]

Christmas Community should use SQLite rather than PouchDB. This may fix #6.

[ndlanier]

As a suggestion, it may be beneficial to allow for an externally hosted sql database to be pointed to and disable whatever db is built in (whether that is sqlite or pouchdb) via an environment variable. This would allow for scalability and would also solve an issue with the helm chart. Currently via the helm chart, if you have to kick the pod/restart the container you need to replace the pod (so it terminates the pod before spinning up a new one) otherwise there will be access issues with the db because it's locked to the instance that is terminating and must wait till the lock is removed.

[Wingysam]

Technically you can specify an external CouchDB instance with DB_PREFIX but I don't think anyone does that. I think the migration should just fail if you have a custom DB server, since the feature is so obscure I doubt anyone that isn't me knows about it.

Re. external SQL server:

• Giving the application full control over all persistence reduces complexity and allows easier migrations.
• Scalability beyond 20 or maybe 500 people shouldn't matter, since this is intended to be used by a family.
• There may be use-cases I haven't thought of which involve having 100k users, for example if someone makes a SaaS for this, especially after #7.
• Supporting SQLite, external Postgres instances, and not other DB engines seems like a reasonable middle-ground between simplicity and scalability to me.