we should never enable js for email testing

Bug report:

If `Vanity.use_js!` is enabled, email a/b testing never adds any participant.

The fix: add a helper that disables the js participant adding that wraps the js.

Note:

This isn't thread safe. I don't think I know vanity well enough to know if turning `@use_js` in the playground into a thread local variable would be a bad thing (so I'd love some feedback on this).