Allow guests to upgrade their accounts

synapse/api/auth.py

synapse/handlers/auth.py

synapse/handlers/register.py

synapse/handlers/room.py

synapse/handlers/sync.py

synapse/rest/client/v2_alpha/register.py

synapse/rest/media/v1/thumbnail_resource.py

synapse/storage/prepare_database.py

synapse/storage/registration.py

synapse/storage/schema/delta/28/upgrade_times.sql

tests/api/test_auth.py