Make striped progress bars its own class

Fixes #13347.

Basically for some stupid reason I scoped striped progress bars to an immediate parent class. That's just silly, and this gets around that by adding a second selector to target just `.progress-bar-striped`. Essentially, just add it to an existing `.progress-bar` and you get stripes overlaid.

Probably need to add some docs for this and perhaps deprecate the old shit.

/cc @tristanlins