mark subscriptions as unsubscribed

[frisi]

instead of simply deleting them.

this is a major change in how subscriptions are handled and requires
quite a lot of changes.
the benefit is, that we can track unsubscriptions now

• we know when they happened (date is included in the csv export)
• we can skip unsubscribed users when re-importing users from an
  external data source

.. ATTENTION:: requires an update of collective.singing too

this fixes #15

[frisi]

@dnouri could you please have a look at this one?

i might add handling for the unsubscribed for the xxx sections in browser.subscribe.py (eg

collective.dancing/collective/dancing/browser/subscribe.py

Line 246 in 397317f

# XXX handle unsubscribed here too?

) and additional tests for them.

before or after merging i'll remove the pin for the collective.singing branch.
i can't run all singing tests via collective.dancing's test-runner (import errors for plone.testing) and i won't have the resources to fix that.
i'd say we're fine when collective.dancing tests pass.