RFR: 1683: The force push notifier needs an overhaul

[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.

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Commit messages:
 - delete two blank lines
 - SKARA-1683

Changes: https://git.openjdk.org/skara/pull/1426/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1426&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1683
  Stats: 307 lines in 8 files changed: 197 ins; 109 del; 1 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