Conditionally update a local copy of the ACE rates page

This adds a new script, get_mta_ace_rates.py, to bring a new copy
of ace.html into the arc data directory only if that file has the
AVERAGE lines that we want to parse.  By conditionally updating a local
copy, we can use the time-based job watcher to only warn when this file
has gone more than expected without a valid update.