Silence

Implements silence endpoints requested in https://github.com/Contegix/cloudykangaroo/issues/17 and re-enables the silence check/client button.

A prompt is displayed when the user clicks a silence button to enter an integer between 1 and 72, with a default of 8. This represents the duration for which the silence stash should exist, in hours, and sets the expires field in the call to the Sensu API. An error confirmation is given if the input is >72 or is not an integer. 

To make this useful, additional development will be required to filter the events list based on existing silence stashes, similar to the Sensu server dashboard.