Add view decorators for checking membership, authorship and admin rights

The view decorators are used to check if a user is a member of a community, admin of the community and/or author of such models as Resource, CommunityPage, etc.
For instance, to make a change to a model instance, the user should be a member of the community and have the specific permission, so the following decorators will be used in combination with `permission_required`.

Dependent on #9 (models) and #15 (`mock` dependency should be moved to `requirements-dev.txt`).
Minor nitpick, but I would add single ticks around "None": `"The lookup value can't be 'None'."` just to add clarity.