Improve the solutions module

[PhilippMatthes]

Fixes: #308, #309, #310, #311.

See the meta issue #311:

• Add a test case which tests for archive access permissions.
• Delete solution archives after associated solution was deleted #309: Add a post delete signal listener, which removes solution archives after the corresponding solution was deleted.
• Verify that solution files are deleted correctly after the corresponding solution was deleted #310: Add a test case, which verifies that archives and solution files are deleted correctly after their corresponding solution object was deleted.

[martinmo]

• one commit per issue please
• Raise PermissionDenied if user tries to download another user's solution #308 is now marked as wontfix, but the test cases you provided should be kept (but adapted to check for 404s)

Changes were separated into 3 unique commits. Changes to the solution archive download view were reverted, but the corresponding test was corrected, so that it looks for a 404 on access violation.

I replaced the calls as proposed and also removed the todo, as it was no longer applicable.