Re-add historical link's to timespans

These were removed as part of the post tweaks patch, but
we don't yet have a new location for the diff links so
add them back.  An overhaul to the history pages is coming
soon from design to replace this in its entirety.

Change-Id: I5944364726109b23d3e1a7980b85db862ca37e43
Merge "Re-add historical link's to timespans"