RFR: Add pull request prune functionality

[Robin Westberg]

Hi all,

Please review this change that adds another mode to the bridgekeeper bot that closes PRs after a configurable period of inactivity.

Best regards,
Robin

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Commits:
 - 83c41425: Add pull request pruner bot

Changes: https://git.openjdk.java.net/skara/pull/247/files
 Webrev: https://webrevs.openjdk.java.net/skara/247/webrev.00
  Stats: 640 lines in 7 files changed: 420 ins; 218 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/247.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/247/head:pull/247

PR: https://git.openjdk.java.net/skara/pull/247