Merged Front contacts cause problems

[rgalanakis]

When contacts are merged or deleted, if the deleted contact is the one the backend is associated with, our code errors trying to update a non-existing contact.

Instead, we should use the phone number as the handle to update the contact.

• Use the presence of frontapp_contact_id to determine if a user's phone number has ever been sent to Front. We CANNOT assume the ID is valid anymore.
• Change FrontappAttributes#contact_id to use the alt handle, like alt:phone:+12223334444
• In FrontappAttributes#_update_contact and _create_contact, use the alt handle.
• In FrontappAttributes#_create_contact, if an error Frontapp::ConflictError is raised, then save a frontapp_contact_id of the error's message (just the whole message, we don't care about the value of this field anymore), and fall back to calling _update_contact. This will happen when the contact already exists in Front.
• In FrontappAttributes#_update_contact, if an error Frontapp.NotFoundError is raised, then clear out frontapp_contact_id, and fall back to calling _create_contact. This will happen when a phone number is changed and it does not exist in Front.
• Change ListSync to use the alt id. You will need to select members, rather than select_map.