Make the queue more visible in the UI

[bennofs]

The interface does not clearly show that dump requests are queued after creation. We should make this more prominent.

Additionally, we should prevent adding the same dump twice to the queue (it is redundant). Since the queuing is not visible, this can happen if after not noticing that your dump is already queued, you try to create it again. In this case, an error should be displayed that explains how the queuing works.