add template code for the dumps secondary download index.html

The page shown at http://dumps.wikimedia.org/backup-index.html
is generated by the monitor script, currently using the template
progress.html which is also used for index.html per dump.  The
monitor script will shortly to be changed to use a separate
template, i.e. this one.

Change-Id: I1b326fb4602600be46c5ca09758fa1ce05f02375