Making link unfurling optional

[bewuethr]

Link unfurling results in edits of the PR description by the Jira bot, which sometimes has unintended side effects:

• The default commit message, if set to "Use PR title and description", contains the very long links from the bot edits, cluttering up the git logs
• Some automations that exclude, e.g., dependabot PRs get confused because the actor on the PR edit event isn't dependabot, triggering checks we don't want to trigger for dependabot PRs (this is fixable on our end, but still a nuisance)

We have set up GitHub autolinks everywhere, so the link unfurling is effectively redundant for us; all in all, it would be great if we were able to just disable it. I understand this is already what happens for GitHub Enterprise Server if the corresponding permissions are missing; it would be nice if this could be made opt-out for non-server instances.