fix: detect mergify is enabled with new mergebox

[MattSturgeon]

The <a href="/apps/mergify"> element is not present in the new mergebox, however the mergify icon is present in both mergeboxes.

Fixes #29 (for real)

[mergify]

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[MattSturgeon]

Interestingly this fix isn't always needed.

If the @mergify app has commented on the PR, then document.querySelector('a[href="/apps/mergify"]') will match the icon next to the comment.

Therefore, even without the fix, I see the icons added in this PR, because of #32 (comment).

However if the app has not commented on the PR, there will be no matching <a> tags, since the new mergebox does not wrap the status-check app-icons with links.

Regardless, explicitly checking if the icon is present on the page is more robust and handles both mergeboxes, regardless of whether comments are present.