Make sure that tasks that will not override existing overrides refuse to enable and provide an error message saying why.

#528022 by tha_sun: Change wording of "Add variant" to "Create variant" to avoid confusion with the "Add variant" tab.

#522836 by andrewlevine: Add form_clean_id() to ctools_rebuild_form() to prevent extraneous -1s from appearing.

name, they would accidentally share data.