Reworked the magic link stimulus controller, because the system test
was causing double-submission of the form (because the event was
bubbling up). I think that change simplifies the form and will still
work well for iOS devices.
When typing a magic link, rather than pasting it, iOS seems to send
`keydown.enter`, and we weren't submitting the form in that case.
Changed to submit the form in either case, and use the state of the
input target to help guard against double submissions.