Zoom/pan tweaks

[edemaine]

• [x] Remember the view of each page, initially the 1:1 origin view
  • [x] In particular, when going to a new page, reset view
  • [x] Store in localstorage
• [x] Add "return to origin" button (replacing/augmenting the "zoom to 100%" button)
• [x] Limit zoom in or out to avoid overflow/NaNs
• [x] Zoom out has a performance issue with grids; see #21 for automatic grid scaling.