RFR: mlbridge: support generating JSON for webrevs

[Erik Helin]

Hi all,

please review this patch that add support to the mlbridge bot for generating JSON versions of webrevs. Repositories can
now be configured to generate HTML webrevs, JSON webrevs or both. I added an additional unit test for verification.

Testing:
- [x] `make test` passes on Linux x64
- [x] Added unit test for new functionality

Thanks,
Erik

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Commit messages:
 - mlbridge: support generating JSON for webrevs

Changes: https://git.openjdk.java.net/skara/pull/719/files
 Webrev: https://webrevs.openjdk.java.net/skara/719/webrev.00
  Stats: 377 lines in 6 files changed: 309 ins; 9 del; 59 mod
  Patch: https://git.openjdk.java.net/skara/pull/719.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/719/head:pull/719

PR: https://git.openjdk.java.net/skara/pull/719