Add treatments (for A-B tests etc.)

Treatments allow administrators to assign numerous users with badges.
In order to select the users that should apply, the administrator selects them by badges themselves.
The admin can then assign automatically generated treatment badges to these users.
Among each group the treatment badges are assigned randomly, but with the same amount for each treatment badge.

For example, possible source badges may be:

* Party members (10 users)
* Non-party members (21 users)

The administrator could then proceed to create a treatment "first-experiment" that could assign the users as follows:

* 5 party members get stuffed in group A and B each
* 10 non-party members get stuffed in group A, and 11 in group B.

Of course, this is only useful if we do more with the badges afterwards, for example use them to selectively send more/less/different emails via the mass messaging service, or different presets (hhucn/adhocracy.hhu_theme#121).
In the future, we could also add additional badge behaviors, like color schemes, font sizes, number of interface components, etc.

See hhucn/adhocracy.hhu_theme#110 for details on the big picture.
Fix typo and simplify treatments
Merge pull request #396 from phihag/treatments

Add treatments (for A-B tests etc.)