use protocol relative url for image links on stats homepage

https://stats.wikimedia.org/ contains mixed content. It loads images
from http://upload.wikimedia.org. So I replaced all occurrence of
"http://upload.wikimedia.org" with "//upload.wikimedia.org", and
"http://wikimediafoundation.org/favicon.ico" with
"//wikimediafoundation.org/favicon.ico".

Change-Id: I31ef9823a5cb3bd92f49bb7f8ba06555a57c5321