Rename applies_to_exp to visibility_expression

applies_to_exp is unused and we can use it for storing
MiqExpression which can say whether the button will be
displayed or not.

db/migrate/20170713083836_rename_applies_to_visibility_expression.rb

@@ -0,0 +1,5 @@
+class RenameAppliesToVisibilityExpression < ActiveRecord::Migration[5.0]
+  def change
+    rename_column :custom_buttons, :applies_to_exp, :visibility_expression
+  end
+end

db/schema.yml

@@ -985,7 +985,7 @@ custom_buttons:
 - guid
 - description
 - applies_to_class
-- applies_to_exp
+- visibility_expression
 - options
 - userid
 - wait_for_complete