Add completion markers to cards

[Vidalia]

format_tiles has an option to display a completion percentage in the tile, showing the number of activities in that section that have been completed. It could be useful if we included something like this in format_cards as well.

• Add a section completion percentage on cards
• Allow users to toggle the completion marker on and off
• Allow administrators to set a default visibility for the completion marker
• Different styles for the marker? (Percentage completed vs "x / y completed"?)