Attempt 2: Notifications in the composebox

This moves the notify-not-in-view notifications into the composebox area.
It also tries to be a bit smarter about what action it links and what it displays.

(imported from commit 1c79bd0d9ef972059a006b17501a09b72e961ee3)

static/templates/compose_notification.handlebars

@@ -0,0 +1,5 @@
+{{! Content of sent-message notifications }}
+<div class="compose-notifications-content">
+    {{note}} {{#if link_class}}<a href="#" class="{{link_class}}" data-msgid="{{link_msg_id}}">{{link_text}}</a>{{/if}}
+    <button type="button" class="out-of-view-notification-close close">×</button>
+</div>