Use input mask on datetime fields

[moveson]

We got an error resulting from a user innocently providing a datetime string in format mmddyyyy hh:mm:ss instead of the expected mm/dd/yyyy hh:mm:ss.

We should implement input masking to avoid this issue.

• Convert existing inputmask_controller.js to use vanilla JS
• Input mask on event group entrant create modal
• Search for other places where input masking may be helpful

Reference: https://opensplittime.sentry.io/issues/4176897735