This is permanent, if a stream item is updated it doesn't re-appear on
the dashboard. This is done by marking the stream item instance as
hidden, which means adding the hidden field to the two indexes.
Change-Id: Ifa544cef4ea2d46f4214e8317af4c9e3baed76eb
Reviewed-on: https://gerrit.instructure.com/3603
Tested-by: Hudson <hudson@instructure.com>
Tested-by: Selenium <selenium@instructure.com>
Reviewed-by: Zach Wily <zach@instructure.com>