Major refactor of build setup

* Gets rid of `debug/`. All debug builds go in the `build/` directory.
  This makes things substantially simpler.
* Compiles asset groups as they are pulled in by pages.
* More verbose.
* Only recompiles templates when needed.
* Only concatinates assets when requested. The extension
  versions really don't want this functionality.
* The firefox background code may now be modified without recompiling
  (although the browser still has to be restarted).
Fix description on firefox extension.
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