Add a re-Start button in the Scavenge view

[riccardone]

There is a new feature to start and stop the Scavenge operation.

This PR add a Cancel button https://github.com/EventStore/EventStore.UI/pull/195 that can be used to stop the running Scavenge.

We can implement a Start action/button beside a stopped Scavenge. We need to retrieve the completed chunk number and then add the button doing the http POST request http://localhost:2113/admin/scavenge?startFromChunk=