LP#1570909 User activity purge function

LP#1570909 User activity purge function

Utility function for removing all activity entries by activity type,
except for the most recent entry per user.  This is primarily useful
when cleaning up rows prior to setting the transient flag on an
activity type to true.  It allows for immediate cleanup of data (e.g.
for patron privacy) and lets admins control when the data is deleted,
which could be useful for huge activity tables.

Signed-off-by: Bill Erickson <berickxx@gmail.com>
Signed-off-by: Chris Sharp <csharp@georgialibraries.org>
Signed-off-by: Mike Rylander <mrylander@gmail.com>