RFR: Initial version of SyncLabelBot

[Robin Westberg]

Initial implementation of a bot that handles the hgupdate-sync label. Initially it will only log its intended actions, so we can verify it while still running the old implementation in parallel.

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Depends on: https://git.openjdk.java.net/skara/pull/1002

Commit messages:
 - Initial version of SyncLabelBot

Changes: https://git.openjdk.java.net/skara/pull/1003/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1003&range=00
  Stats: 379 lines in 13 files changed: 372 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/skara/pull/1003.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1003/head:pull/1003

PR: https://git.openjdk.java.net/skara/pull/1003