Communitymaterial

This MR implements the CommunityMaterial subclass for Material. For communitymaterial, the person who creates it can define view and change permissions on a group and user basis.

Additionally, we removed the groups m2m-relationship between activity (working/project group) and a django user group. Instead, each working group automatically gets an ActivityGroup and members in an activity are automatically added to this group. The name of the ActivityGroup for a given working group, let's say WG SUPTECH is then something like Members of WG Support and Technical Issues (SUPTECH) and it can be used to control the permissions

furthermore we add several more information for uploaded material, including digests (md5 and sha256), file_size and a description field