Fixes #4360 

Also rename: AllowedRenewals -> PotentialRenewals to avoid confusion of
future readers. (An entry in `AllowedRenewals` is not enough to allow a
renewal, the assignment also has to be complete, which is only checked
afterwards.)

- [x] Does not work with renewals as is - fix.
- [x] More tests
- [x] PR docs

Edit 1:
(Relevant blog post:
https://grillapp.net/12935/agile-coretime-pricing-explained-166522?ref=29715)

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Co-authored-by: eskimor <eskimor@no-such-url.com>
Co-authored-by: Dónal Murray <donal.murray@parity.io>
Co-authored-by: command-bot <>