Rich permission controls using a ReBAC/Zanzibar approach

[Floppy]

Resolves #2192.

Uses our new Caber gem to provide a simple ReBAC implementation that doesn't need a separate auth server, unlike the existing ones. Might not be the fastest thing in the world, but optimisation comes later!

[qlty-cloud-legacy[bot]]

Code Climate has analyzed commit d6e360c6 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 86.7% (0.2% change).

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