Billwerk entity types field administration tabs not shown

@Grevil: If you should have any idea... but I guess it's just an internal Drupal route bug in core somewhere. Don't waste too much time here.

Fixed the problem! This was caused by an incorrect base route definition in "modules/billwerk_subscriptions_entities/billwerk_subscriptions_entities.links.task.yml". See the core user module, where this is handled similiarly.

Here is a screenshot of the new output:

I also changed the config forms to use the correct base class. Not a big deal, as there are currently not in use, but still necessary for future implementations. Please review!

Very simple, straight forward changes. Merging them, so I can begin with the code style fixes.

Letting this issue on "Needs review", as if @Anybody has anything he doesn't agree on, we can push a follow up commit.

Great work, thank you @Grevil!! :)

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