Add ability to preview schema templates

**Job Story**

When _creating or editing a schema template_, I want _to preview how the template will look and render before applying it to a policy_, so I can _verify the structure, fields, and sub-schema order are correct without having to apply it live first or navigate to the manage schemas tab._

**Acceptance Criteria**

* Users can trigger a preview of a schema template from the template editing/management screen
* The preview reflects the current saved (or in-progress) structure of the template, including sub-schemas and their order
* Preview shows the template as it would appear when filled out or issued, not just the raw schema definition