Allow Value of "0" (immediately) for the "Purge users who [...] for" config fields

I guess 0 would mean "immediately"?

Please review!

• I needed to overhaul the Settings Form a bit, here are the changes:
• Removed numeric value validation -> Use 'number' form type instead of 'textfield' for automatic validation instead.
• Removed empty value field validation -> Make the value fields required through states api, as soon as the relevant checkbox is checked.
• Change value fields schema from "string" to "integer" with an appropriate update hook added
• Outsource duplicate config code in its own "setConfigFromForm()" method

Please review!

I also tested it manually.

Quite large changes, but tests are passing and this LGTM. Let's merge it.

Automatically closed - issue fixed for 2 weeks with no activity.