WIP: Change slot/container/asset schema, no models
WIP: Change slot/container/asset schema, no models

- Volume is no longer a container.
- The "clip" concept has be raised to the slot level: slot is now a "clip" of a container.
- Assets are largely unchanged, but in the future asset and asset_link may be combined to enforce single-ownership of assets.
- Random POC support functions, currently unused, may be removed.

databrary/databrary,databrary/databrary,databrary/databrary,databrary/databrary