Special newsletter user permision implemented for Dizkus

This is an example how can we implement special permissions (related to issue #8).

In same time it fixes the problem for forums - they are very sensitive in this issue, for example it is possible to have forum for administrators, and now topics in such a forum will not be exposed in the generated newsletter.

It is possible all (or most) of plugins to refactor in similar way (in future, not right now).