First stab at responsive elements using JSS and theme file

Created by: joeadams04

Opening a PR here so we can review the architecture. Once agreed I can make my way through various pages/components and make them fluid/responsive.

For screen-widths of 960px and under, the sidebar should shrink in width and decrease the font-size of the list items.

The theme file holds the values, which are imported via withStyles like usual, and the component itself holds the breakpoint logic to apply the styles.

We use the material-ui way of breakpoints, instead of manually applying media queries, for example @media (min-width: 1px). More info here