Expose scan template level control scanning options

* `ScanTemplate#control_scanning?`
* `ScanTemplate#control_scanning=`
I think I'd just go ahead and remove it. I could see a workflow where an admin goes through an modifies the templates first before flipping the global switch.

As to names, let's make sure we are using the singular "control" then (except in the XML, where there's no choice). Don't need to rework anything not in this commit, but worth auditing at some point.