UFFields bound to phone_ext should clearly handle phone_type_id

While working on the new form designer (CRM-11493), I examined the handling of location_type_id and phone_type_id in the existing form designer. There appears to be some inconsistencies around the handling of "phone" and "phone_ext" fields.

• The "phone" requires setting UFField.phone_type_id, but "phone_ext" does not. This means that it's ambiguous which DB record (which row in civicrm_phone) will store the "phone_ext" value.
• Email discussion with Lobo suggested that 'phone_ext' should only be included in a profile if combined with the corresponding "phone". CRM-10799 suggests that it is valid to have "phone_ext" data without "phone" data. (These aren't strictly contradictory – Lobo's point deals with desirable form design, and CRM-10799 deals more with data design.)

The solution needs to specified. Some possibilities:

A. Require that UFFields for "phone_ext" specify the "phone_type_id"
A.1. Update/test old form designer (CRM_UF_Form_Field)
A.2. Update/test new form designer (org.civicrm.profilemockup)
A.3. Update/test form runtime (CRM_Profile_Form_*)
B. Don't allow UFFields which specify "phone_ext." Instead, automatically include "phone_ext" with any "phone."
B.1. Update/test old form designer (CRM_UF_Form_Field)
B.2. Update/test new form designer (org.civicrm.profilemockup)
B.3. Update/test form runtime (CRM_Profile_Form_*)