Update the padding used by One Click Postage for new layout

70px matches the new tumblr layout and should also work for the old layout which needed at least 10px.

This fixes the keyboard shortcuts D/Q/R/T which currently operate on the previous post (hidden under the header) when navigating with J/K because tumblr changed its padding.

Ideally this parameter would be taken from the `Tumblr.KeyCommands.scroll_offset` variable (plus a little padding) but I don't see a nice way of grabbing that value from inside an XKit extension.