Organization details page made admin only

Added a custom view for the OrganizationDetail view of django-organizations that uses the AdminRequiredMixin to prevent access of non-admin users to the page. Non-admin users accessing /organizations/(id)/ will get a blank white page with "Sorry, admins only'.

Test for this in organization_test.py.

Kanban ticket:
https://aptivate.kanbantool.com/boards/5986-econsensus#tasks-1388749