Adds model for instance admin announcements

bookwyrm/context_processors.py

@@ -4,4 +4,7 @@ from bookwyrm import models
 
 def site_settings(request):  # pylint: disable=unused-argument
     """include the custom info about the site"""
-    return {"site": models.SiteSettings.objects.get()}
+    return {
+        "site": models.SiteSettings.objects.get(),
+        "announcements": models.Announcement.active_announcements(),
+    }