Add snippet placeholder

[piemonkey]

Overview

Add snippet placeholder nodespec and view so they are rendered correctly in RS editor. Required to use rdfa aware snippet insert component.

connected issues and PRs:

binnenland.atlassian.net/browse/GN-4782
Uses: lblod/ember-rdfa-editor#1188
And: lblod/ember-rdfa-editor-lblod-plugins#419
RB part: lblod/frontend-reglementaire-bijlage#241

Setup

You'll need to get templates from the latest RB in order to have snippet placeholders, so need to change regulatoryStatementEndpoint and regulatoryStatementFileEndpoint to point to local and run the RB stack.

How to test/reproduce

In RB, publish a template with a snippet placeholder, and verify that in GN you can select it and insert a relevant snippet.

Challenges/uncertainties

I don't know if it was a conscious choice not to use the rdfa aware version of the snippet insert component, but I assumed it was just an oversight.

Checks PR readiness

• UI: works on smaller screen sizes
• UI: feedback for any loading/error states
• Check cancel/go-back flows
• Check database state correct when deleting/updating (especially regarding relationships)
• changelog
• npm lint
• no new deprecations