Support automatic bridge retrieval using Moat

#15064 (closed) was supposed to track that but it’s been repurposed to integrating in Tails the subset of that work that was ready when we prepared Tails 3.5. This ticket is meant to track everything that’s left on https://trac.torproject.org/projects/tor/query?component=Applications%2FTor+Launcher&sponsor=Sponsor4 i.e. automatic retrieval of bridges (moat).

What moat gives is the “request a bridge” button in Tor Launcher, which appeared in Tor Browser 8.0. When pressed, after solving a CAPTCHA, one gets bridges from BridgeDB automatically.

Feature Branch: wip/feature/15331-moat

User research findings

• During #18762 (closed), P1 and P2 mentioned not feeling safe sending an email to get bridges, especially using Gmail, because they understand that both things could be connected.