Come up with a better terminology for bridges

Our terminology for bridges is confusing:

• Private bridges are bridges that BridgeDB doesn't know about. Users may mistakenly conclude that if a bridge isn't private, it must be public, which is incorrect. Suggestions for other terms: unshared, exclusive, unlisted, unknown.

• Default bridges are part of Tor Browser. Conceptually, default bridges are more like obfs4-enabled guard relays. Suggestions for other terms: built-in (we may have been using that term occasionally), standard, public.

• We don't have a consistent term for bridges that are distributed by BridgeDB/rdsys. Perhaps we don't need a term because that's the default?

How can we improve the situation?

Copying @cohosh, @antonela, @arma, and @gus.