FIX: allow moderators to create categories setting broken

Mods require visibility to everyone group cause category dialogs need to
know about this.

If the site setting `allow moderators to create categories` will not function
without this

Note there is no security expansion of rights here, the group is technically
empty anyway and it always looks exactly the same on all discourse instances

app/models/group.rb

@@ -290,7 +290,7 @@ class Group < ActiveRecord::Base
     # way to have the membership in a table
     case name
     when :everyone
-      group.visibility_level = Group.visibility_levels[:owners]
+      group.visibility_level = Group.visibility_levels[:staff]
       group.save!
       return group
     when :moderators

spec/models/group_spec.rb

@@ -221,9 +221,9 @@ describe Group do
   end
 
   describe '.refresh_automatic_group!' do
-    it "makes sure the everyone group is not visible" do
+    it "makes sure the everyone group is not visible except to staff" do
       g = Group.refresh_automatic_group!(:everyone)
-      expect(g.visibility_level).to eq(Group.visibility_levels[:owners])
+      expect(g.visibility_level).to eq(Group.visibility_levels[:staff])
     end
 
     it "ensures that the moderators group is messageable by all" do