Only apply BC-category ACL in backend, not in frontend.

Only apply BC-category ACL in backend, not in frontend.

Before the category ACLs, the authorid of a category indicated who can WRITE to entries, but this was now used to indicate also who can read entries in a specific category. But of course that old BC should apply seamlessly when upgrading.

Hard to describe, nobody probably reads this anway. Thanks a bunch to Sebastian Bergmann for debugging this with me. :)