plan visualization: set the height in proporation to the width

Previously, we would calculate and the height in absolute pixels based
on figure size and dpi, without taking into account the zoom level of
the plan.

Fix #196. Note that the description there is slightly erroneous, it was
more that height was being dramatically over-set for wide plans.
editor: also resize divs containing plans when emptying them

Addresses another part of #196.
graph.js: redo the svg height setting

I hope this is easier to read and better matched what @domoritz
requested in
https://github.com/uwescience/myria-web/pull/197#issuecomment-54386528.
Or at least is better explained.
editor: switch the panel collapse settings

By default, make the logical plan collapsed and the physical plan shown.
First, this is usually correct. Second, this fixes a bug where the size
of the SVG was not being accurately updated because the panel was
closed.
graph.js: try again

I think I figured what @domoritz meant by letting zoom do the work.
Revert to just setting absolute height of the main container elements.
Merge pull request #197 from uwescience/ui-js-fixes

Ui js fixes