move base config options to ConfigMap and move db migration/connection parameters to pixelfed.db section

[jessebot]

Moving to a ConfigMap helps with readability and also a bit of the issues mentioned in #17 (comment) in case we want to switch to another docker image in the future that requires a .env file.

Two parameters have also moved from pixelfed.db_apply_new_migrations_automatically and externalDatabase.connection. New locations are under the pixelfed.db section:

 pixelfed:
   db:
    # -- options: sqlite mysql pgsql sqlsrv
    connection: pgsql
    # -- Automatically run [artisan migrate --force] if new migrations are detected.
    apply_new_migrations_automatically: false

[jessebot]

oh! hah! I didn't realize you already checked in a configmap @holysoles 😅
https://github.com/small-hack/pixelfed-chart/blob/main/charts/pixelfed/templates/configmap.yaml

Going to remove that one because we now have two :P