Allow users to kick themselves (#3157)

As per the spec:
https://spec.matrix.org/v1.7/rooms/v10/#authorization-rules

"If membership is leave"
->
"If the sender matches state_key, allow if and only if that user’s
current membership state is invite, join, or knock."

I.e. a user can kick themselves. Bridges use this to make a user leave
while giving a reason.

Some recent change (likely
8ea1a11105
but I'm not 100% sure) changed that behaviour, resulting in heisenbridge
being unable to make users leave while giving a reason.
This works fine on Synapse.

Signed-off-by: Timo Rothenpieler <timo@rothenpieler.org>
Co-authored-by: kegsay <7190048+kegsay@users.noreply.github.com>

clientapi/routing/membership.go

@@ -181,18 +181,6 @@ func SendKick(
 		return *errRes
 	}
 
-	pl, errRes := getPowerlevels(req, rsAPI, roomID)
-	if errRes != nil {
-		return *errRes
-	}
-	allowedToKick := pl.UserLevel(*senderID) >= pl.Kick
-	if !allowedToKick {
-		return util.JSONResponse{
-			Code: http.StatusForbidden,
-			JSON: spec.Forbidden("You don't have permission to kick this user, power level too low."),
-		}
-	}
-
 	bodyUserID, err := spec.NewUserID(body.UserID, true)
 	if err != nil {
 		return util.JSONResponse{
@@ -200,6 +188,19 @@ func SendKick(
 			JSON: spec.BadJSON("body userID is invalid"),
 		}
 	}
+
+	pl, errRes := getPowerlevels(req, rsAPI, roomID)
+	if errRes != nil {
+		return *errRes
+	}
+	allowedToKick := pl.UserLevel(*senderID) >= pl.Kick || bodyUserID.String() == deviceUserID.String()
+	if !allowedToKick {
+		return util.JSONResponse{
+			Code: http.StatusForbidden,
+			JSON: spec.Forbidden("You don't have permission to kick this user, power level too low."),
+		}
+	}
+
 	var queryRes roomserverAPI.QueryMembershipForUserResponse
 	err = rsAPI.QueryMembershipForUser(req.Context(), &roomserverAPI.QueryMembershipForUserRequest{
 		RoomID: roomID,