cubicweb-forum #2963508 Basic forum setup [validation pending]

Have a Forum entity type, that can contain ForumThread instances that can be answered through comments.

The forum list should display their topic, number of threads and last activity date.

The forum primary view should display at least all threads' description and number of answers.

done in0.1.0
load left0.000
closed by#7955f963f041 Add Forum and ForumThread primary views.
patchAdd Forum and ForumThread primary views. [applied]