Bug 49557: Fix talk topic add overlay

* Run _super to ensure that parent is opened on a close
* invalidate cache so that when the talk button is clicked on again it
loads the new talk page content
* Hide the button bar when save is clicked to prevent duplicate edits
with the same content

Change-Id: I6775efa41e098c015af30c63c68af8f66f58fc0f
Merge "Bug 49557: Fix talk topic add overlay"