Ensure specs for account and entry case can be generated in a single
invocation of the shared example. Prevent overriding `let` in a loop
causing only the last case to be used in specs.

* Fail if unknown option is passed.

* Allow using `of_entry_or_its_account` instead of passing both
  `of_entry` and `of_account` options to make specs more readable.

* Allow testing only the `forbid` case, by aliasing the `but_forbid`
  option and skipping specs for the `allow` case.

* Make spec description generation more DRY.

The previous implementation also allowed entry publishers as long as
they were also publishers of at least one (possibly unrelated)
account.