Edit UI: use off-the-shelf validation tools

## TODO
- [ ] Prototype the use of an off-the-shelf validation library using CourtOrderPoa
- [ ] Select a well-documented validation API with plenty of examples so new team members can get up to speed quickly
- [ ] Hide validation errors while users are filling out the form
- [ ] Check validation state and show errors (if necessary) when "submit" button is pressed; don't let user final submit
- [ ] Allow progressive form elements with minimum of configuration

Consider the following tools:
- [ ] Vuetify Forms
- [ ] VeeValidate
- [ ] Vuelidate

## What is the problem?
At present, edit-ui uses a mixture of v-forms and custom validation code to validate forms prior to form submission. Currently, the forms show validation errors as soon as you make a mistake (or entry).

## What is the impact?
Form validation is notoriously tricky. As the complexity of our application increases, we need a flexible and extensible validation system that's well documented and easy to use. We need a tool that allows components to be validated dynamically. If users select a form option that yields subsequent questions (called progressive disclosure) our validation system must be able to accommodate additional questions without having to change the parent logic.

## Proposed solution
Use an off-the-shelf validation API that's well used, well documented and gives us the flexibility we need.

Start in the edit-ui area around CourtOrderPoa and review how v-forms and custom validation currently handle submission. Compare Vuetify Forms, VeeValidate, and Vuelidate against the listed requirements; done means selecting and prototyping one API that delays errors until submit, supports progressive fields, and is well documented.

Written by the indexing model from the issue text.