added check so that users can only add themselves to the queue

I just looked over the code for joining queues, and we didn't have proper authorization which meant that any user could added to a queue by simply calling the API even when logged out. This PR adds a basic check so that any user can only add themselves to the queue.

This most likely fixes https://github.com/mvk13ogb/qwait/issues/148, where logged out users would be able to add themselves to the queue.

Warning: haven't tested the code, at least it compiles.