disable Subscribe to Plan button on submit

prevents click-happy users from creating duplicate subscriptions
```disable-on-submit``` adds a spinner if the form is submitted by any button, which can lead to weird behaviors if there are multiple submit buttons (to be fair, this wouldn't happen as the form is currently set up here because the only other button is to cancel, not submit)

for example: https://github.com/dimagi/commcare-hq/pull/3844