Correctly syndicate to configured targets

[paulrobertlloyd]

Fixes an issue where the syndication endpoint was looping over all configured syndication targets and comparing their URLs to that provided in a mp-syndicate-to property, but without accounting for the fact that this value could be an array.

This lead to all syndication targets being syndicated against, even if not selected by the author when creating a post.

Now we loop through each value in mp-syndicate-to, check that a target for that value has been configured, and that the post has not already been syndicated to that URL.

Hopefully this makes syndication a little less buggier, but this part of the application remains more challenging than it perhaps should be.