Correct 500 error on topics#show and topics#destroy

[lfzawacki]

The methods show! and destroy! were invalid in the topics controller, they were probably intended to be used with a gem like 'inherited_resources' but instead of adding it I just rewrote the methods without it. I included some tests for the controller to make sure I'm not breaking anything.

Also while running the tests I found that 'db:seed' failed because the data was an invalid devise user, so I just made a little correction to the data there.