Update tenant and fallback_sip_account on existing phones

[spag]

Since commit 9101b20 the Phone model uses "tenant" and "fallback_sip_account" to store fallback values for "phoneable" and "sip_account".

If a user logs out from a phone, "phoneable_id" is set to the value of "tenant" and "phoneable_type" to the string "Tenant".
"fallback_sip_account" is used in various provisioning controllers as a fallback for the "phone_sip_accounts" table.

Newly created phone entries should populate the fields automatically but after an update from a previous GS5 version those fields will be NULL.

The goal is to create a migration (or something else) to populate "tenant" and "fallback_sip_account" with the right values affecting existing phones only.