Update labels to include a base class

* Builds on the new button styles by requiring a `.btn-default` for the
regular, gray labels
* Updates components examples to show new stuff
Remove dupe hover background color
Merge branch '3.0.0-wip' into bs3_alerts_default_class

Conflicts:
	dist/css/bootstrap.min.css
Merge pull request #9123 from twbs/bs3_alerts_default_class

Update labels to include a base class (fixes #8752)