Add more features flags and default values to `system_settings.py`

Given that Sal's 'Settings' wiki page states the following...

There are defaults set in sal/system_settings.py, but they can be overridden if you choose

... I figured that it might be a good idea to enumerate most every toggle-able feature within the server_settings.py file. That said...

This PR suggests the following changes to sal/sal/system_settings.py:

Add comment explanation (pulled from the wiki page) above EXCLUDED_FACTS definition
Add IGNORE_FACTS setting with default value of []
Add ADD_TO_ALL_BUSINESS_UNITS with default value of False
Add ADD_NEW_MACHINES with default value of True

salopensource / sal