Allow contact numbers to be enabled and disabled

Summary: Depends on D20008. Ref T920. Continue fleshing out contact number behaviors.

Test Plan:

• Enabled and disabled a contact number.
• Saw list, detail views reflect change.
• Added number X, disabled it, added it again (allowed), enabled the disabled one ("already in use" exception).

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T920

Differential Revision: https://secure.phabricator.com/D20010