RFR: Backport security level should be inherited from main issue

[Erik Joelsson]

This is a change Robin prepared, but we need to deploy it ASAP to match the new behavior in the backport plugin after the Jira upgrade. The security level of a backport should simply be based on the security level of the primary bug.

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Commit messages:
 - Backport security level should be inherited from main issue

Changes: https://git.openjdk.java.net/skara/pull/1110/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1110&range=00
  Stats: 25 lines in 5 files changed: 7 ins; 12 del; 6 mod
  Patch: https://git.openjdk.java.net/skara/pull/1110.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1110/head:pull/1110

PR: https://git.openjdk.java.net/skara/pull/1110