Allow resizing sidebar

[kylesferrazza]

Let the user set the size of the expanded sidebar.

https://github.com/STRML/react-resizable may be useful

[kylesferrazza]

Setting

resize: horizontal;
overflow: auto;

on the sidebar is already promising - just need to work out how to disable animations while the user is resizing.

[kylesferrazza]

May be easier to just have three presets: small, 20%, and 50%.

[blerner]

(As it currently stands, using max-width and min-width seem to cause non-smooth transitions; sticking with width alone seems smoother.)