Instead of implementing all details of an user, rely on Django's codebase and
just extend, where absolutely necessary.

So, MiniUser now inherits AbstractUser, but still applies the custom
MiniUserManager.

In MiniUser, the email field is redefined to make it unique (if set) and to
include a verification flag.

Fun fact:
Django automatically registers a receiver for the user_logged_in signal and
updates a 'last_login' field on the active AUTH_USER_MODEL, even if Django's
default User doesn't even have that field!

So, I didn't notice that *my* own callback wasn't even executed, though the
last_login-field got updated during manual testing.

Currently, the test regarding the app's callback is skipped, because I could
not get it working using the mock-library and Django's test-client.

Now, let's start some TTD. How should signup react with different settings?

MiniUser means: only request minimum information from the user. So, if no
valid email address is required, don't ask the user about it.

Made sure to call the model's clean() where needed to apply some everytime validations

Regarding commit c8b6eed:
Ok, I found out, where Django defined last_login by default, so no more need
to provide it as an own field in our custom model. Get rid of it and all
associated code.

Closes #9

Ok, in fact, no Exceptions are probably best.

This means, no database request will be made IOT send mails. But it relies on
a sane project configuration.

Yeah, now it behaves consistently across bulk and non-bulk methods and is testable.
Coverage back up to 100%