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
(and various other lockfile upgrades. "npm audit" is clean.)
The date picker to use with 4.0 is unmaintained, and the new one is
apparently quite different.
Given that 5.0 is expected to be released this quarter, I prefer to
jump straight to the new one and get things working with it, even if
it means using an alpha in the short-term.
