Remove user's avatar URL and displayname when deactivated. (#8932)

changelog.d/8932.feature

docs/admin_api/user_admin_api.rst

synapse/handlers/deactivate_account.py

synapse/handlers/profile.py

synapse/rest/admin/users.py

synapse/rest/client/v2_alpha/account.py

synapse/server.py

synapse/storage/databases/main/profile.py

tests/handlers/test_profile.py

tests/rest/admin/test_user.py

tests/rest/client/v1/test_login.py

tests/rest/client/v1/test_rooms.py

tests/storage/test_profile.py