RFR: 218: Enforce check on approval of CSRs

[Erik Helin]

Hi all,

please review this patch that adds a CSR bot. The CSR bot allows a reviewer to
issue the command `/csr` in a pull request comment. Once issued, the PR cannot
be integrated until the CSR for the issue associated with the PR is in state
"Closed" and "Approved". A reviewer can cancel this requirement by issuing the
command `/csr unneeded` in a pull request comment.

Testing:
- Added a bunch of new unit tests for the `/csr` command and the CSR bot
- `sh gradlew test` passes on Linux x64

Thanks,
Erik

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Commits:
 - 76d62dd8: Add the CSR bot

Changes: https://git.openjdk.java.net/skara/pull/385/files
 Webrev: https://webrevs.openjdk.java.net/skara/385/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-218
  Stats: 928 lines in 10 files changed: 927 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/385.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/385/head:pull/385

PR: https://git.openjdk.java.net/skara/pull/385