RFR: 223: Make required re-reviewing configurable

[Robin Westberg]

Hi all,

Please review this change that makes it possible to configure (per project) if additional commits to a pull request renders previous reviews invalid or not. It also tweaks the behavior of the `rfr` label slightly - instead of dropping it when a PR is reviewed and receives the `ready` label, is will remain for as long the PR is eligible for further reviews. This should reduce the number of label transitions when this feature is enabled.

The change also contains a bit of refactoring to make it easier to add more project specific configuration in the future.

Best regards,
Robin

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Commits:
 - ef0825c9: Initial implementation

Changes: https://git.openjdk.java.net/skara/pull/376/files
 Webrev: https://webrevs.openjdk.java.net/skara/376/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-223
  Stats: 368 lines in 27 files changed: 208 ins; 47 del; 113 mod
  Patch: https://git.openjdk.java.net/skara/pull/376.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/376/head:pull/376

PR: https://git.openjdk.java.net/skara/pull/376