ValidatedTextInput: automatically default label to the label in the field's yup schema

[mturley]

Currently, to get nice-looking validation errors we have to repeat all our field labels in two places: in the schema and on the form field itself.

Looking at the TS types for yup, we should be able to grab the label set in the schema's label() method by looking at schema.spec.label: https://github.com/jquense/yup/blob/master/src/schema.ts#L42

We can probably use this in ValidatedTextInput to automatically provide a default for the label prop so it will render the schema's label if the consumer doesn't pass one in.