Allow to specify security level

This is something I've had on my mind for quite some time but I've never created an issue because I don't know if it's wise to do it UX-wise. However, with the increased interest in disaster communication and the upcoming research on multi-hop social mesh and public mesh, I thought I can at least share this idea here.

Similar to Tor Browser's security settings, it might be cool if users could specify their security settings in Briar, too. Tor Browser's settings could translate to Briar like this:

• Standard (I would rather call it "disaster" or "insecure"): public mesh
• Safer: multi-hop social mesh
• Safest: single-hop social mesh like at the moment

I can see, though, that this is both difficult to implement and difficult to understand for users, but I'm still curious what UX experts like @elioqoshi think about it. If we don't do security settings but want to provide a solution with public mesh, I think forking Briar is the only viable alternative we have.