This requires a bunch of package name changes. See
[https://mui.com/blog/material-ui-is-now-mui/] for their rationale.
[https://mui.com/guides/migration-v4/] lists the changes:
```
@material-ui/core -> @mui/material
@material-ui/system -> @mui/system
@material-ui/unstyled -> @mui/core
@material-ui/styles -> @mui/styles
@material-ui/icons -> @mui/icons-material
@material-ui/lab -> @mui/lab
@material-ui/types -> @mui/types
@material-ui/styled-engine -> @mui/styled-engine
@material-ui/styled-engine-sc ->@mui/styled-engine-sc
@material-ui/private-theming -> @mui/private-theming
@material-ui/codemod -> @mui/codemod
@material-ui/docs -> @mui/docs
@material-ui/envinfo -> @mui/envinfo
```
We only use a few of these.
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
