PhoneNumber's allow_* columns are inappropriate

[jace]

The PhoneNumber model introduced in #1572 had a late change introducing three additional columns: allow_sms, allow_wa and allow_sm. These columns are poorly thought out and may need to be removed:

1. These are preference columns and need to be reset when a phone number is released from an exclusive lock. However, there is no signal handler for such a release, only one for zero reference counts. Since no reset happens, obsolete preferences are imposed on the next owner.

2. These columns do not indicate availability on WhatsApp and Signal Messenger, only explicit preference to use them. However, such preference is useless without availability, so downstream uses (such as User.transport_for_whatsapp) are depending on a misleading preference column.

3. There is no record of who set these flags, or when.

4. Choosing to block SMS is a legitimate preference and has no additional dependency (unlike WA/SM availability), but this should be called block_sms, not allow_sms, and likely should be on UserPhone, not PhoneNumber.