Publicly advertise bridged accounts and rooms

Nobody has claimed this yet.

(part of my secret masterplan for awesome bridging and leveraging existing networks)

I want to be able to use matrix while my friends still use their IM service of choice.
I want to be able to send them a matrix.to link instead of worrying about which messenger they prefer.

For easy discovery, we need to have standard ways of doing the following:
1. [ ] Allow bridges to automatically advertise the bridged account on the identity server for [bridged] -> [mxid]
2. [ ] Allow bridges to automatically advertise the bridged account in the profile for [mxid] -> [bridged]
3. [ ] Enable matrix.to (and other messengers) to read and display public profile information.
4. [ ] Same thing goes for rooms! There should be a standardized way for rooms to advertise publicly what they're bridged to. (maybe even for non-public rooms)
5. [ ] And we probably need a room directory search on the web for people who don't yet have a matrix account, but who might want to join in on the fun through bridged channels.

Start by reviewing the Matrix specification areas covering identity servers, profiles, room directories, matrix.to links, and bridge discovery. Map the five requested discovery mechanisms and determine the standardized protocol behavior needed for bridged accounts and rooms; done means the scope and interoperable requirements are agreed.

Written by the indexing model from the issue text.