Support designating a contact number as "primary"

Summary:
Depends on D20010. Ref T920. Allow users to designate which contact number is "primary": the number we'll actually send stuff to.

Since this interacts in weird ways with "disable", just do a "when any number is touched, put all of the user's rows into the right state" sort of thing.

Test Plan:

• Added numbers, made numbers primary, disabled a primary number, un-disabled a number with no primaries. Got sensible behavior in all cases.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T920

Differential Revision: https://secure.phabricator.com/D20011