Configure system proxy in Kali

I decided to change proxying to be at the system level so that any scripts I wrote would also go through Burp (and not just Firefox requests).

1. Change the Firefox proxy settings from 'Manual' to 'System proxy settings'
2. In Kali, go to the settings for your network settings (e.g. 'Wired')
3. Change proxy to 'Manual', fill in details as before
4. Close and re-open apps so they become system-proxy-aware (the new $http_proxy won't be in running apps' environments)

Some apps might complain about the Burp cert, since it won't match what is expected.  In the case of PyCharm, you can get it to ignore these errors, but I'm not sure whether all apps would be so forgiving.  I might have to change back to per-app proxying in the future.