Calculator variables are hard to grab

[James-Crean]

The text element that displays the variables in the calculator are currently drag and droppable. This works, but also means that one letter variables only have one letter worth of space to grab on to. We should create a wrapper element, or otherwise increase the size of the area that users can grab onto.

While we are at it, some css when hovering wouldn't be a bad idea to help indicate that the variables are interactive.