RFR: pr: add hint about 24h exposure for PRs with certain labels

[Erik Helin]

Hi all,

please review this patch that allows the PR bot to show a hint in the integration message that the PR should perhaps
bit out for review for at least 24 hours if the PR has certain label(s).

Testing:
- [x] Added a unit test
- [x] `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - pr: add hint about 24h exposure for PRs with certain labels

Changes: https://git.openjdk.java.net/skara/pull/846/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=846&range=00
  Stats: 128 lines in 11 files changed: 119 ins; 0 del; 9 mod
  Patch: https://git.openjdk.java.net/skara/pull/846.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/846/head:pull/846

PR: https://git.openjdk.java.net/skara/pull/846