Use monochrome checkmark for API key note

The current green icon is very distracting, and very out of place in Quantum. Would you be interested in switching to a monochrome icon that is more similar to the ones Firefox itself uses in its menus? Could be something like the [checkmark from Font Awesome](http://fontawesome.io/icon/check/), or [any other icon set](https://www.flaticon.com/search?word=check&order_by=1&color=1).