Move settings and their defaults to conf.py (using django-appconf).

The Setting model's .get() and .extent() functions now don't take a
default anymore, rather if a Setting isn't found they get their
defaults from normal Django settings -- a setting called "mysetting"
uses the LIZARD_MAP_DEFAULT_MYSETTING_SETTING setting. Setting setting
setting.

Several defaults were updated to what lizard5-site sets in its
base.py, so the settings can be removed there. No other settings were
actually set in lizard5-site, so how configurable do these things need
to be?