Move admin API to a new prefix

docs/admin_api/account_validity.rst

docs/admin_api/delete_group.md

docs/admin_api/media_admin_api.md

docs/admin_api/purge_history_api.rst

docs/admin_api/purge_remote_media.rst

docs/admin_api/register_api.rst

docs/admin_api/user_admin_api.rst

docs/admin_api/version_api.rst

synapse/app/homeserver.py

synapse/config/server.py

synapse/rest/__init__.py

synapse/rest/client/v1/admin.py