RFR: 975: Add /open pull request command

[Erik Helin]

Hi all,

please review this patch that adds the `/open` pull request command to allow pull request authors to re-open a closed pull request.

Thanks,
Erik

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Commit messages:
 - skara-975

Changes: https://git.openjdk.java.net/skara/pull/1123/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1123&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-975
  Stats: 239 lines in 3 files changed: 238 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/1123.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1123/head:pull/1123

PR: https://git.openjdk.java.net/skara/pull/1123