This is a surprisingly complex upgrade. Some relevant changes from
[their CHANGELOG](https://github.com/mui-org/material-ui/blob/v5.0.0-beta.3/CHANGELOG.md):
* @material-ui/core/styles no longer re-exports some stuff from
@material-ui/styles
* there's no more defaultTheme, so tests need to provide one
* select's onChange has a new type; match that. I haven't actually
tried that the string form (apparently from autofill) works correctly.
* checkboxes no longer default to the secondary color; explicitly
request this in some places.
* checkbox no longer has a checked argument; use event.target.checked
instead.
* date pickers have switched to the new style system, so I had to
redo how I was overridding their spacing for desktop.
* LoadingButton now has a loading property, not pending
* createMuiTheme is no createTheme
