Support Delete Channel action

Move the channel "Settings" action into a "Channel" drop-down in the CM's toolbar (edit mode).

Based on the global enable/disable setting of the channel delete feature (HSTTWO-3728), and if the current user has admin privileges, add a second "channel action": "Delete". Clicking it should open a modal confirmation dialog, warning the user that he's about to delete the current channel. The user can then choose between Cancel and Delete.

(Using the REST endpoint for deleting a channel is part of a separate task.)