As guest users can't add Tags an informing message should appear or the + icon disabled

Steps to reproduce:

1. Access the XWiki instance as a guest user (not logged)
2. On any page click on the plus [+] icon next to "Tags"

Expected results: A message informing the guest user he is not allowed to add tags should appear or the icon should be disabled.

Actual results: The log in box appears at the bottom of the page with some red lines above and below.