Support v3 hidden services

Tor's v3 hidden service protocol has several security improvements over the legacy v2 protocol. We should make a planned migration from v2 to v3 hidden services:

1. Add support for connecting to v3 hidden services (depends on #1279 (closed))
2. Create v3 hidden services for all new and existing Briar accounts
3. After a reasonable time, remove support for v2 hidden services

Each account will have two hidden services during the transition period. This will increase our bandwidth usage, but most of the bandwidth cost comes from downloading the consensus, which will be shared between the two services.