extensions.torbutton.https_proxy and similar preferences are gone

After #15852 (moved) landed on master (commit 1b41636c9351b46bbcb2094a65c65a4407b60a37) extensions.torbutton.https_proxy and friends are no longer. I am getting now:

NS_ERROR_UNEXPECTED: Component returned failure code: 0x8000ffff (NS_ERROR_UNEXPECTED) [nsIPrefBranch.getCharPref]

while the SSL Observatory code is trying to access extensions.torbutton.https_proxy etc.

This part needs to get adapted accordingly.