- Add announcements view using JSX to the dashboard sidebar
- Create a new maintenance interface to edit and manage announcements
- Adds an override to main.html template to include new skip links
- Add plugins required for announcements to TinyMCE

This is motivated by a desire to have system wide messages for students
that show on the dashboard. Enabled with FEATURES['ENABLE_ANNOUNCEMENTS'].
Global staff are allowed to edit from the studio maintenance view.