RFR: 1818: Get rid of extra REST request in GitLabMergeRequest#author

[Zhao Song]

Currently, in GitLabMergeRequest#author, the bot makes an unnecessary REST request to retrieve the HostUser object. 
However, with recent advancements in GitLab, it's possible to parse the HostUser directly from the pull request JSON. 
Getting rid of this redundant REST request would also solve the 'user is inactive' issue.

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Commit messages:
 - SKARA-1818

Changes: https://git.openjdk.org/skara/pull/1471/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1471&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1818
  Stats: 7 lines in 1 file changed: 0 ins; 6 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1471.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1471/head:pull/1471

PR: https://git.openjdk.org/skara/pull/1471