RFR: Allow targeting non-open issues with the label command

[Robin Westberg]

Add a warning to the PR body instead if a non-open issues is targeted (unless the PR is marked as a backport).

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

Commit messages:
 - Allow targeting non-open issues

Changes: https://git.openjdk.java.net/skara/pull/943/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=943&range=00
  Stats: 81 lines in 3 files changed: 77 ins; 4 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/943.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/943/head:pull/943

PR: https://git.openjdk.java.net/skara/pull/943