Additional tests for #625

Adds a test for the specific behavior described in #625. This checks that
toggling a story's ``on_homepage`` flag doesn't affect the lates stories
list.
Sort projects by published date

In the Project list view, sort the projects in reverse chronological order
by their published date/timestamp instead of the last-edited date/timestamp.

This avoids the issue described in #625 where the toggling the ``on_homepage``
field updates the ``last_edited`` field and changes the sort order.