Take out the underscore restriction from state events

It's not needed anymore, and we should remove it while we're in the area.

Includes other misc changes to the schema layout.

event-schemas/schema/core-event-schema/state_event.yaml

@@ -1,19 +1,7 @@
 allOf:
 - $ref: room_event.yaml
+- $ref: sync_state_event.yaml
 description: In addition to the Room Event fields, State Events have the following
   additional fields.
-properties:
-  prev_content:
-    description: Optional. The previous ``content`` for this event. If there is no
-      previous content, this key will be missing.
-    title: EventContent
-    type: object
-  state_key:
-    description: A unique key which defines the overwriting semantics for this piece
-      of room state. This value is often a zero-length string. The presence of this
-      key makes this event a State Event. The key MUST NOT start with '_'.
-    type: string
-required:
-- state_key
 title: State Event
 type: object

event-schemas/schema/core-event-schema/sync_room_event.yaml

@@ -26,8 +26,7 @@ properties:
     description: The globally unique event identifier.
     type: string
   sender:
-    description: Contains the fully-qualified ID of the user who *sent*
-      this event.
+    description: Contains the fully-qualified ID of the user who sent this event.
     type: string
   origin_server_ts:
     description: Timestamp in milliseconds on originating homeserver

event-schemas/schema/core-event-schema/sync_state_event.yaml

@@ -27,7 +27,7 @@ properties:
   state_key:
     description: A unique key which defines the overwriting semantics for this piece
       of room state. This value is often a zero-length string. The presence of this
-      key makes this event a State Event. The key MUST NOT start with '_'.
+      key makes this event a State Event.
     type: string
 required:
 - state_key