Add endpoint for getting current read receipt location

Nobody has claimed this yet.

Currently clients must perform a `/sync` to get the read receipts for a room. This can be disastrous on performance when your client only cares about getting the read receipt for a specific user in a specific room.

It would be more beneficial to lightweight clients (bots, iot) if the homeserver provided a way to look up a user's read receipt (probably their own, in most cases). For a sample use case, a bot may be interested in whether or not it has already replied to a message because it does not have long-term storage of its own.

Possible endpoint: `GET /_matrix/client/r0/rooms/!myroom:domain.com/user/@travis:t2l.io/receipt/m.read`
Possible response:
```
{
"event_id": "$whatever:domain.com"
}
```

Start by reviewing the proposed GET endpoint and its sample response in this issue, then check the surrounding Matrix client-server specification for how room, user, and receipt identifiers are defined. Done means the specification has an agreed endpoint and response for looking up a user's read receipt without performing /sync.

Written by the indexing model from the issue text.