Delete channel

Upon successful validation (HSTTWO-3731) of the front-end's request to delete a channel, and upon successful event dispatching (HSTTWO-3732), the back-end shall delete the specified channel. The following artefacts of the channel shall be deleted:

1) the channel's HST configuration (preview and live, unless the latter is inherited from at least one other channel).
2) the channel's hst:site node
3) the channel's hst:channel nodes (preview and live)
4) all mount points of that channel (which have been verified to be leaf nodes in the hst:hosts configuration)
5) any hst:virtualhost parent nodes of the deleted mount points, if they are now leaf nodes themselves.

If all goes well, all above changes, along with potential custom changes made during the event handling, should finally be saved to the repository as a last step.