RFR: Add support for marking PR body content with an expiration time

[Robin Westberg]

For information in the PR body that is calculated using data from external sources (like Jira, another personal fork,
etc.) we need to perform a periodical refresh, as our normal way of detecting updates cannot be used (the updatedAt
field of a pull request).

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Commit messages:
 - Add support for marking PR body content with an expiration time, for data that comes from "external" sources

Changes: https://git.openjdk.java.net/skara/pull/862/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=862&range=00
  Stats: 143 lines in 7 files changed: 133 ins; 0 del; 10 mod
  Patch: https://git.openjdk.java.net/skara/pull/862.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/862/head:pull/862

PR: https://git.openjdk.java.net/skara/pull/862