Gracefully handle module being disabled and re-enabled

Right now the language_content_type_$type variables are left in an inconsistent state when this module is disabled, since no other modules know what to do with the value ENTITY_TRANSLATION_ENABLED. ET should reset each content type using entity translation to not use translation, and users should be warned about this change. However, disabling and then re-enabling the module should be a no-op, so a list of content types using entity translation needs to be stored on disable and then used to restore the content type settings on re-enable (with users warned that these settings were altered). This stored information should be completely wiped on module uninstall. However, on the UX principle of "least surprise", if the translation settings for a content type were changed between when the module was disabled and when it was re-enabled, no action should be taken because we assume the user made the change on purpose and wants it that way.