Adds a check icon for dropdowns

Adds a big-old check mark to the icon list.

It's used for things like dropdowns:

Previously we used font awesome in these dropdowns, but since we're trying to move away from that library, I'm putting the check mark in here so we can use the <icon> component instead.

The check mark itself is the same one used for the success icons, just without all the extra paddings and fancy bits.