Re-introducing the content template feature

The feature works using the OData REST API, this epic is mostly about the install package and the UI.

• add the previously removed /Root/ContentTemplates folder to the default install package with permissions
• add a new GUI section on admin UI where users can create templates

Global templates

Global templates (in the /Root/ContentTemplates folder) should be accessible for regular users on admin UI, not only admins.

Displaying them on admin UI

• A new menu item on the left that simply displays the ContentTemplates subfolder as a tree.
• Optional/later: add a deep link from the content type list to the corresponding template folder.

Local templates - OUT OF SCOPE for now

[Bug] Local templates (e.g. created under a workspace) do not work. This is a backend bug.

Concept docs