Improve stats announcement filtering

This is a whole heap of usability improvements to the interface for managing statistics announcements designed to make it easier to spot announcements that have not yet been linked to a publication.

By default, a user will see their own organisation's future statistics announcements, ordered in ascending order (i.e. the one happening the soonest first):

![screen shot 2014-10-28 at 15 34 58](https://cloud.githubusercontent.com/assets/3687/4811103/20bedfbc-5eb8-11e4-97d0-d4782c70b771.png)

Those announcements without publications are highlighted with the yellow warning. A message at the top of the screen also indicates the number of "imminent" statistics announcements that do not have a publication (imminent in this case meaning within the next two weeks).

The filter has been updated to include a selector for various date ranges ("All", "Past", "Future" and "Next 2 weeks"), and a checkbox for showing only those that do not have a publication associated with them.

* [x] CSS cleanup
* [x] Peer review
* [x] Product owner review

Story: https://www.agileplannerapp.com/boards/173808/cards/5956