RFR: 1587: Adding review comments should not mean approval status change

[Zhao Song]

The issue here is ' if a user leaves additional comments using the "Review changes" dialog, and selects the "Comments", then their approval is considered revoked.'

I believe this happens because if we use "Review changes" to add a comment, it will add a review with `Review.Verdict.NONE` to our pr.  According to our current logic, this new review will cover old reviews, so the approval will be revoked.

I made changes to `CheckablePullRequest#filterActiveReviews`. Now If latest review is just a comment, the verdict will inherit from the previous review.

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Commit messages:
 - SKARA-1587

Changes: https://git.openjdk.org/skara/pull/1399/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1399&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1587
  Stats: 79 lines in 2 files changed: 79 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/skara/pull/1399.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1399/head:pull/1399

PR: https://git.openjdk.org/skara/pull/1399