MDL-43598 modchooser: no more scrolling with the chooser open

Conflicts:
	lib/yui/build/moodle-core-chooserdialogue/moodle-core-chooserdialogue-min.js
	lib/yui/build/moodle-core-notification-dialogue/moodle-core-notification-dialogue-min.js
	theme/bootstrapbase/style/moodle.css