Extend and improve the use of visitContext

- Extend the use of visitContext to all routes so we can stop reading from a PV for serial
- Tidy up the parseInstrumentSession code and its usage
- Have a text field where the visit can be set but with a dialog opening asking for confirmation
- The text field should also be disabled in case no one is logged in (needs the log in to be in a context and after button click instead of as soon as the page opens)