Account for user permissions with ContentTemplate::getPreviewSuggestionQuery

Overview

The current logic in ContentTemplate::getPreviewSuggestionQuery does not account for entities the user might not have access to - such as unpublished ones when the use does not have the permission to view published content.

This can result in \Drupal\canvas\Entity\ContentTemplate::normalizeForClientSide returning a client side representation with a null 'suggestedPreviewEntityId', which in turn results in the UI stating

No preview content is available
To build a template, you must have at least one [entity type].

Despite there being plenty of previewable entities available -- it just happens that the one with the highest entity id is not one the user has access to view.

This can be reproduced by creating a template for the article content type, and make it so the highest-nid article is unpublished

Visit that template and this is what you get:

Proposed resolution

- Expand the query conditions to limit results to ones the current user can view
- Consider adjusting the "No preview content is available" message to not assume an absence of [[entity type]] entities - they might exist yet aren't available for preview for other reasons.

User interface changes