Added a font cards view.

The font cards view will be responsible for displaying the
correct font card depending on the context, and making sure the
card has the correct proportions.
Added two new directives and related LESS files.

    * <ddt-font-card-add> to add a new font family.
    * <ddt-font-card-titlebar> to serve as a font card's title
      bar.
    * common.less contains global styles.
    * fontCardCommon.less contains common styles for font
      cards.
    * directives.less imports all less files associated with our
      directives.