RFR: 1997: Allow the creator of a PR to set the commit author

[Zhao Song]

In this pr, we will introduce a new pull request command `/author`.

If an author wants to contribute a change to a repository, but he doesn't have access, then someone with the access will be able to help him with this command.

Usage: `/author [set|remove] [@user | openjdk-user | Full Name <email at address>]`
Examples: 
`/author set @openjdk-bot`
`/author @openjdk-bot`
`/author set J. Duke <duke at openjdk.org>`
`/author remove @openjdk-bot`
`/author remove`

Note: only Committers of the repository would be allowed to issue /author command.

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Commit messages:
 - update
 - update
 - SKARA-1997

Changes: https://git.openjdk.org/skara/pull/1553/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1553&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1997
  Stats: 358 lines in 7 files changed: 355 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/skara/pull/1553.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1553/head:pull/1553

PR: https://git.openjdk.org/skara/pull/1553