Refactor panels to have a base class for improved flexibility when customizing (and no overriding to start, too)
update customizer to latest vars
Merge branch '3.0.0-wip' into bs3_panel_base_class

Conflicts:
	dist/css/bootstrap-theme.min.css
	dist/css/bootstrap.min.css
Merge pull request #9819 from twbs/bs3_panel_base_class

BS3: Refactor panels to require default class