[Announcements] Manually set order of global announcements

[milesl]

Currently, global announcements are sorted by end date, with the longest running announcement at the top.

As we kick-off our semester start, we're relying heavily on announcements to communicate information to our users, and we often need to balance long-running informational messages with short-term, urgent announcements about service outages.

In order to make the more urgent announcements appear at the top of a user's dashboard, we have to repeatedly delete, and re-post the long-running announcements to move them to the bottom. This creates extra work, and extra opportunities for error when we're already under pressure. (It also causes the announcement to re-appear if someone has previously cleared it.)

Please add the ability to manually control the order of display for global announcements.