Add environment field to sandboxes

[howard36]

Describe your changes

• Part of MOD-3656: Make sandboxes environment-scoped. App-less sandboxes should still be tied to an environment.

Backward/forward compatibility checks

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Check these boxes or delete any item (or this section) if not relevant for this PR.

• Client+Server: this change is compatible with old servers

Note on protobuf: protobuf message changes in one place may have impact to
multiple entities (client, server, worker, database). See points above.

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Changelog

[mwaskom]

LGTM. That resolver object sure knows a lot of things doesn't it :)