feat: add support for Postgres database provider

[stonith404]

With this PR it is now possible to use Postgres instead of Sqlite as the database provider.

The following environment variables were added that allows the admin to configure the database provider:

Variable	Default Value	Description
DB_PROVIDER	sqlite	The database provider you want to use. Currently sqlite and postgres are supported.
SQLITE_DB_PATH	data/pocket-id.db	The path to the SQLite database. This gets ignored if you didn't set DB_PROVIDER to sqlite.
POSTGRES_CONNECTION_STRING	-	The connection string to your Postgres database. This gets ignored if you didn't set DB_PROVIDER to postgres. A connection string can look like this: postgresql://user:password@host:5432/pocket-id.

This PR can be tested with the stonith404/pocket-id:postgres-development Docker image.