Add ability to preview template in post editor for non administrators

[fabiankaegy]

Description

Since the update to unify the PostTemplatePanel in #56817 administrators can preview the current template in post-editor.

Any non-administrator roles (anyone without the edit_theme_options capability) cannot preview the template because they are not allowed to access the template/template part/navigation rest endpoints.

Proposed solution

Open up view-only access to any of the custom post types that are used in the site editor to any other user roles with the edit_post capability.

Additionally some of the blocks such as core/template–part & core/navigation will need to be updated to include a view-only mode. #58037, #58485, #58301

Todos

• Allow view access of template rest endpoint to anyone with the edit_post capability #60317
• Render non-editable preview of template part when user does not have capability to edit template part #60326
• Add a preview only mode to the navigation block #60809
• Enable template preview in post editor for non administrators #60447

[fabiankaegy]

As @youknowriad suggested in #58301 (comment) I started by opening a PR that changes the permission callbacks of the template rest endpoint to allow view-only access to any users with the edit_post capability here: #60317

Once that is merged we can continue in smaller chunks.

[fabiankaegy]

Now that #60317 was approved and merged by a REST API maintainer I have created #60326 to add the preview mode to the template part block.