Draft: Base ui changes 03
Closes #34403 (closed) #40015 (closed) #40016 (closed) #40027 (moved)
#34403 (closed) is the largest change. This patch tries replicating the available browser.privatebrowsing.autostart
pref for Desktop and Fennec. As mentioned in the ticket, there are some trade-offs with how this is implemented and presented.
Test builds are available: https://people.torproject.org/~sysrqb/base_ui_testbuilds/fennecProduction/