Revise onboarding to take new Firefox experience into account

Firefox 68 ESR does not come with the onboarding extension anymore which Mozilla and we used to provide an onboarding experience for our browser based on Firefox 60 ESR. We hacked around that for Tor Browser 9 by reverting a bunch of patches and adjusting the code we had (see: legacy/trac#28822 (moved)) but that strategy won't work forever.

Instead we should look into how the new onboarding in Firefox works and develop our own on top of that. Or: We make a conscious decision to provide onboarding independent of Firefox's one because we don't like the new one Mozilla ships.