Threads: Read receipts & notifications (#1255)

* Spec MSC3771: Threaded read receipts

Note: this builds on a (as of writing) non-existent "threading" section, which is part of a different commit.

* Spec MSC3773: Threaded notifications

* changelog

* Various clarifications per review

data/event-schemas/schema/m.receipt.yaml

@@ -38,6 +38,14 @@ properties:
                                         type: integer
                                         format: int64
                                         description: The timestamp the receipt was sent at.
+                                    thread_id:
+                                        type: string
+                                        x-addedInMatrixVersion: "1.4"
+                                        description: |-
+                                            The root thread event's ID (or `main`) for which
+                                            thread this receipt is intended to be under. If
+                                            not specified, the read receipt is *unthreaded*
+                                            (default).
                     "m.read.private":
                         type: object
                         title: Own User