fix dashboard metric completion on backspace in Firefox
fix dashboard metric completion on backspace in Firefox

Firefox does not seem to trigger the 'keypress' or 'keyspecial' events
for the backspace key, all seems to work correctly with the 'keydown'
event. (Chrome does seem to trigger 'keyspecial' for the backspace key.)

According to MDN, 'keypress' is deprecated, 'keyspecial' is undocumented,
and 'keydown' is well supported.

deniszh/graphite-web,drax68/graphite-web,criteo-forks/graphite-web,criteo-forks/graphite-web,cbowman0/graphite-web,deniszh/graphite-web,criteo-forks/graphite-web,criteo-forks/graphite-web,deniszh/graphite-web,graphite-project/graphite-web,graphite-project/graphite-web,cbowman0/graphite-web,cbowman0/graphite-web,drax68/graphite-web,drax68/graphite-web,deniszh/graphite-web,drax68/graphite-web,cbowman0/graphite-web,deniszh/graphite-web,graphite-project/graphite-web,graphite-project/graphite-web,drax68/graphite-web,graphite-project/graphite-web,criteo-forks/graphite-web,cbowman0/graphite-web,deniszh/graphite-web,criteo-forks/graphite-web,graphite-project/graphite-web,cbowman0/graphite-web,drax68/graphite-web