Announcementboard merge into master

See merge request !1

Changed Announcement.objects.all() to Announcement.objects.order_by('-pub_datetime') so the most recent announcement is at the top of the announcements board.

Added related_name for foreign keys, changed pub_datetime to be an automatic timestamp. Updated views of announcement board to match the necessary format.

Created Announcement and Reaction classes in models.py and fixed an error in urls.py of the main project