User without rights to create categories should not see the "add category" button

If users do not have the right to create categories, they should not see the "Add Category" icon when editing/creating a blog post.

Currently they see the "add category" icon:

but if they try to create a category, they get an error message:

It would be more user friendly if they do not see the "add" icon at all.

This scenario cannot happen for the default blog, as blog entries are in the same level as categories, but can happen for custom created blogs.

The use case is that administrators might limit the right to create new categories to a certain group, because they feel normal users should not freely create new categories.
 

Steps to reproduce:

• as admin, go to wiki administration, "Other" -> "Blog"
• create a new Blog (e.g. "Test") and make sure the categories are stored in Test.Categories
• go to the "Categories" space of the new blog, and limit edit rights to the XWikiAdminGroup
• log out and log in as a non-admin user
• go to the "Test"-Blog, and create a new blog post
• creating the new blog post should work, but trying to create new categories should reproduce the error message above.