Allow prefixing dashboard url and consuming of dead letter queue messages

[sonus21]

• Dashboard URLs used to have prefix /rqueue but when it's used with gateway than static page starts throwing 404 since it can't be directly accessed, similarly for other urls. Now url can be prefixed using rqueue.web.url.prefix spring app configuration.

• Message sent to the dead letter can not be consumed in earlier version of the application, now this can be consumed, a consumer work on that the application must set deadLetterQueueListenerEnabled=true in RqueueListener annotation.

Fixes:
#30
#29

[coveralls]

Coverage decreased (-0.5%) to 93.445% when pulling 68aab55 on fix-29-30 into 1cb7e7c on master.