RFR: Add support for setting all labels in a single request

[Robin Westberg]

Add support for setting all labels of an issue / PR with a single call, and use it for clearing labels on newly created backports.

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Commit messages:
 - Add support for setting all labels in a single request

Changes: https://git.openjdk.java.net/skara/pull/993/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=993&range=00
  Stats: 56 lines in 7 files changed: 54 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/993.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/993/head:pull/993

PR: https://git.openjdk.java.net/skara/pull/993