Make vocabulary lookup use context rather than portal.

Add permission checker which uses field write permission to determine if the
user is allowed to access a vocabulary via JSON API.
Update to current mockup widget code (313be3553d65c5a01ec2ba5da42b01801ad4ab67)
Add ordering support to Select2 multiple select widgets.  On by default for
z3c.form Lists and Tuples.  Off by default for AT Lists.
Add check for whether the vocabulary requested is used for the relevant field/widget when performing AJAX vocabulary permission checks.
Special case related items widget to always request catalog vocabulary from site root.
Merge remote-tracking branch 'origin/contextual-vocabulary'

Conflicts:
	plone/app/widgets/at.py
	plone/app/widgets/browser/vocabulary.py
	plone/app/widgets/configure.zcml
	plone/app/widgets/dx.py
	plone/app/widgets/static/widgets.min.css
	plone/app/widgets/static/widgets.min.js
	plone/app/widgets/tests/test_browser.py