Add dashboard announcements feature

- 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.

setup.py

@@ -68,6 +68,7 @@ setup(
             "bulk_email_optout = lms.djangoapps.bulk_email.policies:CourseEmailOptout"
         ],
         "lms.djangoapp": [
+            "announcements = openedx.features.announcements.apps:AnnouncementsConfig",
             "ace_common = openedx.core.djangoapps.ace_common.apps:AceCommonConfig",
             "credentials = openedx.core.djangoapps.credentials.apps:CredentialsConfig",
             "discussion = lms.djangoapps.discussion.apps:DiscussionConfig",
@@ -83,6 +84,7 @@ setup(
             "user_authn = openedx.core.djangoapps.user_authn.apps:UserAuthnConfig"
         ],
         "cms.djangoapp": [
+            "announcements = openedx.features.announcements.apps:AnnouncementsConfig",
             "ace_common = openedx.core.djangoapps.ace_common.apps:AceCommonConfig",
             # Importing an LMS app into the Studio process is not a good
             # practice. We're ignoring this for Discussions here because its