Preserve saved "viewport" graph extents in URL-encoded links

[melange396]

Make it so the view mode (aka NavMode) is set to "pan" when loading URL-encoded epivis links (those produced by the "Link to this view" button). Currently, the default of "autofit" is applied, which loses the saved graph axis extents (defined by the "viewport" parameter encoded in the URL). However, in the case of missing/unspecified viewport parameters, use the autofit functionality to scale all of the visible signals into view.