Refactor today option in SimpleSearch date selection

This component is getting slightly unwieldy.

• It may be better to reimagine this as "SimpleSearchDatePicker" (Or "TokenisedDatePicker" or something like that)
  • a single field
  • internal controls for translating between tokens and dates
  • dealing with initialValues in a smarter way.

This is more work, but will be a nicer component, and clean up the Form considerably

• Similar approach will be wanted for the Internal Contact UUID filter
• (replaces the current approach of hidden fields to track relative vs absolute)
• Changes needed to initialValues functions etc
• Remove custom css and instead use the Layout component instead

This will allow us to reuse this component as a "relative date selector" elsewhere should we wish to with minimum effort.

We also need to create an RTL test for this component, which might as well wait for this work