RFR: 1856: Include a link to the webrev comment in the pull request's description

[Zhao Song]

Some users complained about that GitHub often hide the webrev comment and it's hard for them to find webrev comment among hundreds of comments. 

In this patch, the pr bot would add the link to webrev comment to the pr body.

Originally, the comments from mlbridge bot would not trigger the update of the pr, now, when mlbridge bot **first** post the webrev comment, it will trigger the update of the pr.

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Commit messages:
 - fix import
 - SKARA-1856

Changes: https://git.openjdk.org/skara/pull/1494/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1494&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1856
  Stats: 73 lines in 6 files changed: 70 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/skara/pull/1494.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1494/head:pull/1494

PR: https://git.openjdk.org/skara/pull/1494