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`).
Renamed the`DashboardTestCase` class to a more specific name. I assume we will have `DashboardFormsTestCase`, `DashboardViewsTestCase` and all of them will require different setup methods.

If the idea is bad and we want to keep all the tests in one class, I will revert this change.