Send Micropub Updates when posts are syndicated

As part of #158 (closed), we want to make sure that posts that are syndicated to other platforms have their syndication link updated.

We only want to update URLs that are present in the syndicate-to configuration, so it doesn't accidentally rewrite other URLs that have been linked to.

This requires making post-deploy a Micropub client itself (for update scope only), as well as exposing the syndication configuration to this service through the new SyndicationUpdateClient.

This also needs to fix a poor assumption around how we receive the URL of the syndicated post, as noted in https://github.com/aaronpk/Telegraph/issues/31#issuecomment-615857548.

Closes #158 (closed).