RFR: 1271: Webrev generation should be optional

[Zhao Song]

Currently, webrevs are generated automatically when a user pushes a new commit, and it's mandatory to choose either JSON or HTML when configuring the mlbridge bot. If both of them are disabled in the configuration, the bot will throw errors.

In this patch, generating a webrev will be optional. If both JSON and HTML are disabled, no webrev will be generated.

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Commit messages:
 - SKARA-1271

Changes: https://git.openjdk.org/skara/pull/1475/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1475&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1271
  Stats: 257 lines in 3 files changed: 240 ins; 0 del; 17 mod
  Patch: https://git.openjdk.org/skara/pull/1475.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1475/head:pull/1475

PR: https://git.openjdk.org/skara/pull/1475