Add drop/purge queue to qless-web

[wodow]

This would be very useful during development with qless.

Currently, I'm using & abusing redis-cli to drop queues.

[dlecocq]

Seems like a reasonable addition. I don't personally have the bandwidth for it at the moment, but the feature in qless-core would look something like this. For all the jobs in all the states in a queue, invoke cancel. The cancel method sends out pubsub messages to all the workers so they should be able to honor that the job is no longer valid.