Module loader

This set of patches make writing external modules possible.

* Modules can be written outside the plinth source tree
  * Installed in plinth.modules namespace package (PEP 420)
  * Installed in the own package and get loaded by Plinth

The changes are:
* Load modules using configuration in /etc/plinth/modules-enabled
* Instead of using the filename for module name, read the contents of the file. This allows for more flexible module naming and any python module can become Plinth module.
* Use Django application loading and simply our module loading
* Some more cleanups (box key related stuff which is not used)
* Update documentation related Python3 switch

This branch is on top of my other two branches and @fonfon 's Django setup fixes.