Room versions 8 and 9: Restricted rooms (#3387)

* Room versions 8 and 9: Restricted rooms

MSCs:
* https://github.com/matrix-org/matrix-doc/pull/3083
* https://github.com/matrix-org/matrix-doc/pull/3289
* https://github.com/matrix-org/matrix-doc/pull/3375

* Changelogs

* Capitalization

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

* Remove verbiage for spaces because they don't exist

* Iterations on text

* Another clarification

* Make error code descriptions consistent

* Apply suggestions from code review

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* Incorporate from merge

* Misc language update per review

* Update accuracy before splitting auth rules

* fix wtf moment

* Fix up v8 and v9 to match "fully specify room versions"

* Scope auth events selection to room version

* Apply consistency

* Add changelogs

* Review part 1

* Apply suggestions from code review

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* Split out redaction sections

* Clarify general case of join conditions

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>
Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

data/api/server-server/definitions/send_join_response.yaml

@@ -55,4 +55,14 @@ properties:
         properties: []
       example:
         $ref: "../examples/minimal_pdu.json"
+  event:
+    type: object
+    title: SignedMembershipEvent
+    x-addedInMatrixVersion: "1.2"
+    description: |-
+      Required if the room version [supports restricted join rules](/rooms/#feature-matrix). The signed
+      copy of the membership event sent to other servers by the resident server, including the resident
+      server's signature.
+    example:
+      $ref: "../examples/minimal_pdu.json"
 required: ["auth_chain", "state", "origin"]