RFR: 2586: Support multi fix version mode in IssueNotifier

[Erik Joelsson]

In some projects, other than the JDK, we would like to use Skara in a simplified Jira workflow, where we use the multi value aspect of the Jira FixVersion field instead of creating backport records. To support this I propose to add a configuration option in the IssueNotifier "multifixversion", with default value false, which corresponds to the current behavior with backports. When set to true, then instead of creating backports, the notifier would just pile on additional fix versions when a commit is integrated into multiple different branches.

In addition to this, I would like to also add handling of a "branch-" prefix for a version strings. This would mimic the current "repo-" prefix which is already in use for JDK projects.

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Commit messages:
 - Correct spelling
 - SKARA-2586

Changes: https://git.openjdk.org/skara/pull/1738/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1738&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2586
  Stats: 141 lines in 7 files changed: 107 ins; 5 del; 29 mod
  Patch: https://git.openjdk.org/skara/pull/1738.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1738/head:pull/1738

PR: https://git.openjdk.org/skara/pull/1738