Room visibility in the directory

Whether a room is listed in the public directory. MRS only indexes public rooms, so if we hold the room the answer is "public"; if we have never crawled it (or it is banned) you get a 404, same as any homeserver would. This is a search engine, not a speakeasy: there is no "private" to report, only "we have it" or "we don't". r0 and v3 share one handler; r0 is the legacy alias.