opt in/out per network

[snarfed]

Not critical now, since we only have a few networks, but as we add more, notably Nostr, I expect people will want to be bridged into some networks but not all.

For example, based on these polls, a (small, unscientific) sampling of fediverse users is more interested in bridging to Bluesky than Nostr.

[snarfed]

Merging into #880, this is how I'm currently building that.