More phone number features

[hmpf]

• Verification of phone numbers (we could reuse the new system to verify email maybe, but less necessary due to fetching known emails from Feide)
• Ensure that phone numbers are unique per user, user + phone number should be unique
• Destinations should have optional labels #344

Important that changing a phone number is possible, since the numbers in the notification-profiles also change then.

[hmpf]

If we switch to multiple email addresses per user we'll need email address verification.

[hmpf]

See #121