RFR: forge: initial support for updating review comments

[Robin Westberg]

Initial implementation of review comment updates for GitHub.

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Commit messages:
 - Support updating commit comments
 - Add support for updating review bodies
 - GitHub support for updating review comments

Changes: https://git.openjdk.java.net/skara/pull/927/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=927&range=00
  Stats: 63 lines in 10 files changed: 61 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/927.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/927/head:pull/927

PR: https://git.openjdk.java.net/skara/pull/927