Add danube_edition option to the Misc section of the configuration file.
Without this option, this 'missing section' test case will fail with
'missing option' instead of 'missing section'. Coupled with the
unpredictable order of reading options, this only happens sometimes.
The name 'Plinth' (cfg.product_name) is not used anymore after my previous
commits.
Reason for the complete removal: I do not think that users should know or have
to care about the internal name of the web interface of the FreedomBox.
* Add unit tests for cfg.py, context_processors.py, and menu.py
* Add new plinth/tests/data directory for miscellaneous test data
* In cfg.py, add an explicit check to verify the existence of the secondary
(non-default) plinth.config file
* In cfg.py, replace deprecated configparser.SafeConfigParser with
configparser.ConfigParser
