Activation and Approval logic must be refactored

From the GH Issue from the old repository: https://github.com/3scale/system/issues/8939

Right now the signup-related controllers (among some other objects) have too much logic specific about approving accounts and activating users and many times it is the same code with slight differences (better explanation in the issue).
For this reason, there must be a refactor with a new unique class with this logic.
I am writing this issue because, as mcassola said, it is something 'pending' since a while ago, that must be done but this conversation keeps popping up every time in a while so it may be good to keep track of this issue in here. And if we keep it in a GH issue from the old repo, nobody will ever look at it again and it will be abandoned.