Refactor the Progress Bar

[stedmanblake]

4 simple styles, applied in order: .default_stage

• Sets up basic styling that applies by default to all elements of the progress bar.

.past_stage

• Applies to any stage at or before the current "active_stage".
  • bottom bar color
  • darker text color

.current_stage

• Always maintains its stage text and icon color

.selected_stage

• Applies to the stage that's "selected", i.e. corresponding to the stage page we're on.