Rewrite progress component without <progress> element

- <progress> element didn't allow animation, labels overlaid, multiple bars, etc.

- Revamps CSS to use something more similar to v3's implementation

- Ditches variant mixin for `bg-` utils

- Rebuilds docs to match, including adding a new Height section for customizing that.

Only potential remaining todo is adding `.sr-only` instances to within the bar. Unsure if that's necessary.

scss/_mixins.scss

@@ -43,7 +43,6 @@
 @import "mixins/list-group";
 @import "mixins/nav-divider";
 @import "mixins/forms";
-@import "mixins/progress";
 @import "mixins/table-row";
 
 // // Skins