document the need (and script the process) to toggle jobs on/off to prevent unneeded rebuilds in stable branch

Max pointed out a flaw in our release process:

after a push to QE, jobs ought to be disabled to prevent unneeded respins, which make plugins LOOK different by virtue of their version number being incremented for no reason (github sources have not changed).

So, I've written a new python script to use the Jenkins api to post enable/disable requests for 1 or more jobs.