R1:596435b35e30

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.

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