RFR: 1994: Reply from /reviewer credit can be clearer

[Zhao Song]

This patch improved the bot's reply in some cases when the pr author issues '/reviewer <user>' command.

When the pr author is trying to credit an user with '/reviewer <user>' command,

If the user already approved this pr, Skara bot should reply "Reviewer <user> has already made an authenticated review of this PR, and does not need to be credited manually." 

If the user already reviewed this pr(but not approved), Skara bot should reply "Reviewer <user> has already made an authenticated review of this PR, but did not approve it. Manually crediting them is not allowed."

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Commit messages:
 - add test
 - SKARA-1994

Changes: https://git.openjdk.org/skara/pull/1547/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1547&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1994
  Stats: 26 lines in 2 files changed: 25 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1547.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1547/head:pull/1547

PR: https://git.openjdk.org/skara/pull/1547