RFR: 1271: Webrev generation should be optional

[Zhao Song]

On Thu, 16 Feb 2023 00:03:32 GMT, Zhao Song <zsong at openjdk.org> wrote:

> 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.

Above `ArchiveMessages#composeConversationFooter`, there is a comment says `// When changing this, ensure that the PR pattern in the notifier still matches`. I tried to figure out where the PR pattern is, but I couldn't find it.

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