Initial commit of Extension:AccountAudit

The extension adds a table for tracking the timestamp of the most recent
login for a user.  This will allow us to determine the (in)activity of a
user and thusly whether or not we can unilaterally rename the user in the
event of clashing local accounts during unification.

Change-Id: I163682c55031e77bba02d67cd62839f9541adc99