Add API for setting account_data globaly or on a per room basis

synapse/api/filtering.py

synapse/handlers/account_data.py

synapse/handlers/message.py

synapse/handlers/sync.py

synapse/rest/client/v2_alpha/__init__.py

synapse/rest/client/v2_alpha/account_data.py

synapse/rest/client/v2_alpha/sync.py

synapse/storage/__init__.py

synapse/storage/account_data.py

synapse/storage/schema/delta/26/account_data.sql

synapse/storage/tags.py