[Bug 881440] Add UI for global announcements

kitsune/announcements has always had the idea of a global announcement, it just was never used. This commit adds a section to the base template to show the templates. It stole the style of the "Your Firefox is out of date!" warning (and I unified the UI for both).

There is also a minor cleanup commit here. It is unrelated and probably inconsequential.

To add these announcements, you have to use the admin. A global announcement is one that does not have a locale or group.

r?