Adding support for creating/updating field presence stream rule types

Unfortunately this stream rule type differs from all the others because
it has no rule value. To disable the value field in the stream rule
dialog I added a very dirty hack to the javascript which just hides
those fields when this stream rule type is selected from the dropdown.
This was done in a hackish way for two reasons:
- We decided to refrain from bigger changes to the 0.20 branch in favor
  of the refactored development branch
- Doing this in a proper way would mean to introduce a dynamic way of
  creating the stream rule add/update form using a dynamic list of
  required attributes. This is possible, but it would mean that we would
  have to change the semantics of the persisted model. Right now we are
  about to refactor all persisted models to a new persistance layer,
  which would make any work to support a behaviour like this obsolete.

Sincerely yours,