Converting Articles to statuses should use id instead of url

[trwnh]

Currently Articles are converted to statuses by taking title, summary, and url. This can cause breakage when the url changes, e.g. in blogs that allow for custom slugs, especially in cases where a user forgets to set the slug when composing the post and adds it a few seconds after the fact. Since Update activities are not supported by Mastodon except on actors, this leaves a broken status cached forever. Using the id instead should be less prone to breakage.