Improve the solutions module

[PhilippMatthes]

This meta issue consists of the following partial issues:

• Raise PermissionDenied if user tries to download another user's solution #308 to improve and test the semantics and security of the solution archive download view
• Delete solution archives after associated solution was deleted #309 to delete the solution archives after the corresponding solution was deleted (and to add tests on this)
• Verify that solution files are deleted correctly after the corresponding solution was deleted #310 to verify that the solution files are deleted after the corresponding solution was deleted