Markdown announcements, announcement editing, announcement notification fixes

[trevnels-cs]

• Update announcement management panel to be more consistent with others
• Consistent announcement spacing in Dashboard header
• Announcements can now contain markdown content
  • Markdown renderer updated to allow inheriting color from parent so that text renders properly inside Callouts.
• Announcements can now be edited after-the-fact
  • Currently these aren't pushed over the socket like newly created announcements to reduce mass socket activity, clients will eventually catch up when navigating/reloading.
• Fix system announcement notifications not being created for offline users
• Optimized away N+1 issues with bulk announcement notifications - replaced per-user db operations with a single bulk operation.
• Notifications now track the ID of their source announcement, so deletes/edits can now properly extend to affected notifications.