Progress bars

Add variables for background colors. Instead of hard-coded (and different) green, red, yellow, and blue colors, we use the new (more semantic) global colors.
Refactor progress bar classes for simpler CSS.
New structure is now .progress on the outer <div> and .progress-bar on the inner <div>.
Instead of placing modifier classes on the parent, they are placed directly on the bars (e.g., .progress-bar-info) in addition to the required .progress-bar class.

See https://github.com/twbs/bootstrap/pull/6342