The URL to request the schema representation is the unique identifier.
We can't default to $request->getURL() since that's different for form submissions.
The schema.schema_url key is redundant, since the identifier is already contained on the top level 'id' key.
Keeping schema_url in a schema itself makes it less portable, particularly once we transition into
generic schemas which are not reliant on a particular record context (that's only contained in the schema_url)
This also fixes the issue of form schemas not refreshing after submit,
e.g. when form fields are added or removed.
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