Dropdown for slider expressions

[mmmaaatttttt]

Per Eli's feedback:

• Rather than using a built-in select element, would be nicer to use a custom react component so that we could use mathquill for expressions that will otherwise look gross
• Rather than having a refresh icon, listen for changes in the calculator and automatically update what displays in the dropdown

[japamat0]

I'd like to own this issue. I think getting the list to update when a new expression is added automatically won't be too difficult so I'd like to get submit this in two PRs.

The first will:

• Get rid of the backslash on extra whitespace in slider expression
• update the dropdown automatically when an expression is added

the second:

• design or find & implement a dropdown menu that allows sub and superscripts for better styling