Front-end validation errors display on some search fields

[GuySartorelli]

For some FormField types (noticed specifically with CurrencyField but bound to be with others as well), there's front-end validation errors that occur on-blur in the search filters in gridfield.

The example screenshot below comes from silverstripe/silverstripe-framework#11602 (review)

Steps to reproduce

• Set up a DataObject with a Currency db field.
• Add that field as a searchable field
• make a model admin for the new class
• Click "Search options", click on the currency field, and then click off it.
• Behold, a message saying the value isn't valid.

Notes

• It's unclear whether we should allow some validation here or not. Perhaps engage the UX team?
  • Removing the frontend validation resolves the problem, technically, but reduces the usefulness of these specific form fields if they're explicitly used
  • Keeping the frontend validation could be useful for some scenarios but isn't backed up with backend validation at all
• You can still submit the form - the error is purely visual
• None of the default scaffolded search fields should have this problem anymore - but it still happens on explicitly set fields like CurrencyField and probably EmailField