RFR: 1683: The force push notifier needs an overhaul [v4]

[Zhao Song]

> A user reported a bug related with the force-push notification. Erik investigated it and found the implementation of this notification should not be in the `PullRequestBranchNotifier`. Also, users preferred to not get force-push notification if the force-push happens when the pr is in draft state.
> 
> In this patch, the implementation of checking force-push and notifying user are moved to `CheckWorkItem`. Also, force-pushing or rebasing a draft pr will not warn user any more.

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-1683
   
   # Conflicts:
   #	bots/pr/src/main/java/org/openjdk/skara/bots/pr/CheckWorkItem.java
 - adjust comment
 - fix some bugs
 - delete two blank lines
 - SKARA-1683

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Changes: https://git.openjdk.org/skara/pull/1426/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1426&range=03
  Stats: 312 lines in 9 files changed: 193 ins; 109 del; 10 mod
  Patch: https://git.openjdk.org/skara/pull/1426.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1426/head:pull/1426

PR: https://git.openjdk.org/skara/pull/1426