Added 'ACLGateway' class which enforces permissions checks (via
'ralph_permission' decorator on 'dispatch' metod) and which should be
present in mro path of every class-based view.

Fixed 'ralph_permission' decorator and added 'decorated_with' property to it
(for faciliating unit tests).

Applied 'ralph_permission' decorator to the function-based views.

Added test checking if every view inherits from 'ACLGateway' class (for
class-based views) or is decorated with 'ralph_permission' decorator
(for function-based views) - both of them are for enforcing permissions
checks. This test includes a list of excluded callbacks and callbacks
decorated manually (i.e. not inheriting from 'ACLGateway' class) - for
the explaination see the comments in the source code.
flake8 fix
Code review fixes ('has_core_access' as default permission to check).
Merge pull request #986 from xor-xor/permissions

Ralph's views permissions review and fixes.