Show .onion icon on Identity drop down?

As a follow-up to #23247 (moved) we thought about showing the respective .onion icon on the identity box as well (instead of a version of the lock icon). See: comment:66:ticket:23247 for the idea.

However, we should deal with the fact that the HTTPS treatment in vanilla Firefox does not necessarily match the lock icons of the URL bar and the identity box 1:1.