Set back to state if alpha2

Problem/Motivation

We decided to create a separate, more straight-forward, less abstract module https://www.drupal.org/project/billwerk_subscriptions → which doesn't rely on subman.

For that reason, we should revert the heavy refactorings here and bring this back to the status of alpha2.

alpha5 should then just be the same as alpha3.

For archive, we could create a refactoring branch before, representing the current state of the module. Perhaps it might be helpful for someone.
I don't think we'll ever invest time here any more afterwards.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes