Teacher homepage: optional override of teacher announcement

[breville]

The "census_announcement" DCDO hash can now contain values specifying the heading, description, analytic ID and URL of the the teacher announcement that shows at https://studio.code.org for teachers. If the "census_announcement" DCDO key is set, then all four of these values will be expected. If the DCDO key is not set, then we fall back to the default announcement.

The full DCDO hash for this special announcement now looks something like this:

DCDO.set('census_announcement', {'homepage_text'=> 'This is a special announcement.', 'homepage_subtext' => 'Announcement subtext', 'alert_heading' => 'Announcement heading', 'alert_text'=> "This is the announcement text.", 'alert_url'=> "https://blog.code.org/12345", 'teacher_announce_heading'=> 'Teacher announcement heading', 'teacher_announce_description'=> 'Teacher announcement description.', 'teacher_announce_url' => 'https://blog.code.org/123456', 'teacher_announce_id' => 'census_announcement'})

[Bjvanminnen]

I wonder if we're better off having a JSON string here than a ruby hash?

[breville]

We use these DCDO flags in a mix of Ruby & JS code. JSON could be handy but I think the native hash is handy too. As it is, this code should only live for about a week.