add zoom/pan

[erosson]

mousewheel or keyboard or +/- buttons to zoom; drag to pan; like gmaps

This looks promising: http://package.elm-lang.org/packages/zaboco/elm-draggable/latest

[erosson]

15 starts this! Currently feature-flagged: https://ch2.erosson.org/?enableZoom=1

before we can enable this by default:

• [ ] zooming and panning interact strangely; panning doesn't keep up with my mouse. should feel the same at any zoom level.
• [ ] zoom goes a bit too fast per mousewheel tick. should zoom a bit slower, for more available zoom levels.
• [ ] Empty margin on the outside is okay, but it should be the same for left/top as it is for bottom/right
• [ ] size seems to be hardcoded - should scale to graph size, like svg size does, to support future characters better
• [ ] using the mousewheel feels a bit like a double-scrollbar. what can we do about that? scale svg to user's screen size?