fix(ew): insert canvas template using value URL

[magdalenaorlowska]

Description

Template rows carry URL in the value column (path is for blocks).

Fix: read item.value, as per DA Setup Library docs and the /edit reference implementation (handleTemplateClick).

No backward compatibility for Template Sheets that might have been created with path to bypass the bug.

Related Issue

Motivation and Context

How Has This Been Tested?

1. Open /canvas#/{org}/{site}/{page}.
2. Click on the document so the editor (window.view) has a cursor.
3. Open the right panel and select Templates (under the "Library" section).
4. Click the + (add) icon on a template.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[hannessolo]

Added this fix in #1021, but maybe we could keep the tests from this PR?