WIP: Reject bridge descriptors posted to non-bridge authorities
Right now if my bridge descriptor gets uploaded to the directory authorities, poof I'm now a public relay, even if I didn't mean to be.
That's not the end of the world, since I am technically offering to be a relay already, and the only difference is that I didn't opt to publish my descriptor myself.
But still it seems like we should make the choice explicit inside the descriptor.