Candidates must use verified phone numbers when applying to jobs

[jace]

Candidates are currently provided a free form phone number field. This should be replaced with a verified number selection field using Lastuser-verified numbers.

Further, to make #91 viable (whether that ticket is implemented or not), the same number should not be used in a job application by another user in the past 30 days.

The UI can be improved to add phone numbers inline as an incremental improvement outside the scope of this ticket.