RFR: 1856: Include a link to the webrev comment in the pull request's description [v4]

[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.

Zhao Song has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains five commits:

 - Merge branch 'master' into SKARA-1856
   
   # Conflicts:
   #	bots/pr/src/test/java/org/openjdk/skara/bots/pr/PullRequestBotFactoryTest.java
 - fix a problem
 - fix some problems
 - fix import
 - SKARA-1856

-------------

Changes: https://git.openjdk.org/skara/pull/1494/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1494&range=03
  Stats: 102 lines in 10 files changed: 95 ins; 1 del; 6 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