
You can add custom icons for each topic's exercise completion badge.

Steps:

1) Add your custom icon in all appropriate sizes and statuses to /images/power-mode/badges.
    ...your filename should match the title of the target topic, and 
    it should be lowercased, alphanumeric, and use underscores instead of spaces.

    Examples for topic "Addition and subtraction":
        40x40px: /images/power-mode/badges/addition-and-subtraction-40x40.png
        60x60px: /images/power-mode/badges/addition-and-subtraction-60x60.png

        40x40px (completed): /images/power-mode/badges/addition-and-subtraction-completed-40x40.png
        60x60px (completed): /images/power-mode/badges/addition-and-subtraction-completed-60x60.png

2) Add the topic name (such as "Addition and subtraction") to the TOPICS_WITH_CUSTOM_ICONS
    list at the end of badges/topic_exercise_badges.py.

    TODO: come up w/ a nicer process for this.

