Updating to latest material-ui alpha (#59)

Several new versions of the material-ui@next have been released since we last updated. These have made a few breaking changes to the their api.

This MR updates the Design System to resolve those issues.

Summary of Changes

• Hidden was removed from material-ui in favour of useMediaQuery
• Various exports have moved, eg:
  • import { Theme } from '@material-ui/core/styles/createMuiTheme'; -> import { Theme } from '@material-ui/core/styles';
• MuiThemeProvider renamed to ThemeProvider
• createMuiTheme renamed to createTheme
• shouldForwardProp renamed to rootShouldForwardProp

Closes #59 (closed)