build: Implement source maps for js build target

We currently concatenate files. This makes debugging a little
harder. Thus, let us implement support for source maps :-)

Also:
* Removed symlinks inside demos/ which conflict with the path
  resolution for source maps. This was a hack anyway (for
  publishing demos on doc.wikimedia.org) which is no longer needs
  that hack.
* Previously the apex/agora theme files did not have header
  (forgot to add intro.css for those in modules.json), this is
  now fixed by having concat prepend the banner automatically.

Note:
* For now css is still without source maps. Coming soon!
* We ran the css files (compiled from less) through concat
  eventough they were single files. This was done only to
  expand VERSION/DATE. This is kind of silly, but might as well
  keep that as-is for now.
* The link to the map is established through a comment that is
  picked up by browsers when their developer tools are enabled.
  This is ignored otherwise as a comment, and stripped away by
  any minifier.

Change-Id: Ie271e806ebf0a0437d42f43e516f8b5ed10a6cf7