Add admin api for sending server_notices (#5121)

changelog.d/5121.feature

docs/admin_api/server_notices.md

docs/server_notices.md

synapse/rest/__init__.py

synapse/rest/admin/__init__.py

synapse/rest/admin/server_notice_servlet.py

tests/handlers/test_user_directory.py

tests/push/test_email.py

tests/push/test_http.py

tests/rest/admin/test_admin.py

tests/rest/client/test_consent.py

tests/rest/client/test_identity.py

tests/rest/client/v1/test_events.py

tests/rest/client/v1/test_login.py

tests/rest/client/v1/test_rooms.py

tests/rest/client/v2_alpha/test_auth.py

tests/rest/client/v2_alpha/test_capabilities.py

tests/rest/client/v2_alpha/test_register.py

tests/rest/client/v2_alpha/test_sync.py

tests/server_notices/test_consent.py