Fix instance size dropdown and launch at capacity.

* Disable and mark unavailable sizes in the instance size dropdown.
* Disable #launchInstance button when we're at capacity.

	modified:   api/occupancy.py
	modified:   resources/js/cf2/utils.js
	modified:   resources/js/cf2/views/new_instance_screen/instance_size_dropdown.js
	modified:   resources/js/cf2/views/new_instance_screen/new_instance_screen.js
Merge branch 'master' of github.com:iPlantCollaborativeOpenSource/atmosphere