#6 General cleanup of the frontend js code

The current js code is a mess. Some specific issues:

• it should use the json API to get information about the panorama to display, instead of using javascript variable generated from a template.

• it is primarily designed for 360° pictures, and does not handle non-360° pictures very well (see issue #1).

• it implements its own tile display logic, while there probably are libraries to do that.

• there are multiple systems of coordinates, with conversions between them:

  • the backend uses absolute pixel coordinates (x/y) to refer to points in an image.
  • the frontend uses relative coordinates (x between 0 and 1, y between -0.5 and 0.5) to refer to points in an image.
  • the frontend also uses screen-based coordinates, in pixels. The relation between screen coordinates and image coordinates depends on the zoom level and the current position in the image, and wraps around for 360° images.
  • both frontend and backend also manipulate azimuth and elevation (but that's the point of celutz)

Also there is jQuery included, but there are some plain old Javascript codes that could be removed by using jQuery utilities (eg. $.getJSON() instead of getXMLHttpRequest()).