Feature/remove database routing add isites lms url

All apps in ext_tools use the same database, so remove the custom db router and app_mappings from each settings file. Also, add in the ISITES_LMS_URL setting for each environment - this is used by the course site wizard in the event that an official course site already exists and it is an iSite.
The ISITES_LMS_URL  is missing from tests.py. I see it added to other environments and test_settings(which is the settings used by unit tests). I see no harm in leaving it in test_settings if that si required, but it should be added to tests.py