Remove article from all issues when unpublished

[J16053]

Expected Behavior

Kind of the opposite of #97, unpublishing an article should remove it from all the issues that it was published in so that it can no longer be accessed. Also related to #340.

Current Behavior

When an article is unpublished using the unpublish button, it stays in all the issues it was previously published in. If that article is then accessed, its Date value will be NULL.

Possible Solution

Check to see which issues the article is published in when unpublishing, and remove all the database entries for that article before unpublishing it.

Steps to Reproduce (for bugs)

1. Select a published article in the admin interface.
2. Click "Unpublish".
3. On the main site, visit the issue that the article was previously published in: the article will still be there.
4. Click on the link to the article, and everything about it will look the same, except where the date should be "Invalid date" takes its place.