RFR: 2088: Skara /reviewers command is no longer effective

[Erik Duveblad]

Hi all,

please review this fix for [SKARA-2088](https://bugs.openjdk.org/browse/SKARA-2088). The fix consists of adding a special value (`disable`) to the key `minimum` in the `[checks "reviewers"]` subsection. This way the functionality enabled by the `minimum` key can be disabled by `AdditionalConfiguration.java` (since INI doesn't feature a way for keys to be unset).

I also made the `ReviewersCheckConfiguration` a bit more strict - the `minimum` key should never be combined with the keys `lead`, `reviewers`, `committers`, `authors` or `contributors`.

Finally I added tests for the above three scenarios.

Thanks,
Erik

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Commit messages:
 - skara-2088

Changes: https://git.openjdk.org/skara/pull/1581/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1581&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2088
  Stats: 132 lines in 4 files changed: 109 ins; 2 del; 21 mod
  Patch: https://git.openjdk.org/skara/pull/1581.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1581/head:pull/1581

PR: https://git.openjdk.org/skara/pull/1581