Introduce ability to filter post listing screen by editorial author

As a result of #9 the "Mine" link on the post listing screen will switch to showing posts that you're an assigned author of. We therefore need to either:

* Convert the "Mine" view into a combo view that shows both posts that you're an assigned author of and the editorial author of (which would mean a combined query for the term and the `post_author` field), or
* Introduce a new filter for viewing posts that you're the editorial author of.

Also, really need to nail the terminology. Too many authors.