Simplify config loading.

These changes should make it easier to customize standard Plinth file
locations for other distributions going forward.  Most of the defaults
have been removed from *cfg.py*: we now assume that your plinth.config
file contains all the data we need.  This may turn out to be a bad
decision (not everybody retains a copy of the original config file).

I've also reorganized *plinth.sample.config* to separate the
directories from the files they're storing.  This should be the file
distributions patch in order to customize file locations per their
preferred file organization.
Remove static self-reference.
Merge simple config changes.
Merged with upstream.