RFR: 2302: Notify when PR is ready for sponsor

[Zhao Song]

This patch is trying to make skara bot be able to send an email notification about the "pr is ready for sponsor" comment.

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Commit messages:
 - SKARA-2302

Changes: https://git.openjdk.org/skara/pull/1663/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1663&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2302
  Stats: 28 lines in 4 files changed: 16 ins; 2 del; 10 mod
  Patch: https://git.openjdk.org/skara/pull/1663.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1663/head:pull/1663

PR: https://git.openjdk.org/skara/pull/1663