Decide between Day.js and date-fns

We currently use Day.js for the calendar export, and date-fns for picking the current term, plus as the underlying date/time library for the [MUI date picker](https://mui.com/x/react-date-pickers/quickstart/)

Using both is wasteful - larger bundle, more dependencies to track and update. We should be able to move to just one.

- Investigate which one is "better" (support/issues, bundle size)
- Replace the removed one with the other