This commits adds two reset buttons to the interface page.
One button allows the reset of all interface counters, namely,
packets and bytes received, packets dropped, and the number of
flows exported and dropped (when mysql or elasticsearch is enabled).
The other button allows the reset of only the drops.
ntopng optionally exports flows to MySQL or ES. This change
keeps track of possible drops (e.g., due to a slow mysql server)
and shows them to the user.
