Extending the limited m.read receipts [Feature]

The spec makes it quite easy to add new types of m.receipt messages. How about we implement:
- [x] A sent receipt (once the message makes it to the sender's homeserver, meaning that the sender's job is done)
- [ ] A reception receipt (once the message makes it to one of the recipient's devices)
- [x] A read receipt

Like WhatsApp's single tickmark, double tickmark and blue tickmarks

Locate the Matrix specification section defining m.receipt messages and compare the existing sent and read receipt entries. Document the reception receipt's semantics for delivery to one recipient device, with the specification clearly defining when it is emitted and what it represents.

Written by the indexing model from the issue text.