This:
- brings consistency - no more mixing `_name_prefix` and `_registry_prefix`
- adds extensibility - a future patch will allow reconfiguring all registry prefixes for all roles in the playbook
We still have `_docker_` vs `_container_` inconsistencies.
These may be worked on later.
Since upgrading mautrix-slack (and pinning to v0.1.0) in e4b54c37fe,
we expect double-puppeting to require the new appservice double-puppeting method.
This commit switches the mautrix-slack bridge to it.