Clean up the announcement date metabox

Now that we allow editing of announcement dates in the admin, things should be cleaned up a bit to be a little friendlier.

* [x] Put date inputs above archive links
* [x] Only list Daily, Monthly, and Yearly rows once. Multiple dates should be comma separated.
* [ ] For extra credit, have a box to add a date so that an announcement could appear more than 3 times.