Svg rendering fixes

Make the bundle/diagram endpoint refer to the dynamic charm/icon one, so that icon rendering is faster and also the default icon is displayed when the charm has no icon.
Also fix the default icon rendering, avoiding new lines at the beginning of the SVG (XML declaration).
Please add https://docs.google.com/a/canonical.com/document/d/1TgRA7jW_mmXoKH3JiwBbtPvQu7WiM6XMrz1wSrhTMXw/edit#bookmark=id.sn07dkq9ijht (or short link)