Users should not be able to customize the skin template from the Administration

To reproduce:

• Go to Global Administration: Themes
• Search for skin in the “Skin” section

Result: The first result is the XWikiSkinsTemplate document, which is also named Skin.

Expected: Since the template should be reused as is, the document should not appear there. Otherwise users will easily select, cutomize and break it, particularly given its name.