Consider making phone number validation mandatory

[jonespm]

Now that we have phone number validation feature we may want to consider making it mandatory. This would eliminate unintentional spam to the phones of users who have an unvalidated number entered.

Since we're on a protected system this hasn't been a problem yet, and I'm not sure what we'd do with every number currently out there that isn't verified. But may be something we'd consider for in the future.

[hemashahumich]

1. Moving forward, it is mandatory to validate the phone number. If the phone number is updated, it must undergo validation.
2. New hosts or attendees have the option to subscribe to notifications.
3. Users have the ability to remove their phone number and save their notification preferences.
4. If no phone number is provided, the checkboxes will be disabled. They will only be enabled once a number is entered and successfully validated, allowing users to save their preferences.