Support setting status message in rooms that look like 1:1s

Part of https://github.com/vector-im/riot-web/issues/1528

src/client.js

@@ -2262,6 +2262,27 @@ MatrixClient.prototype.mxcUrlToHttp =
     );
 };
 
+/**
+ * Sets a new status message for the user. The message may be null/falsey
+ * to clear the message.
+ * @param {string} newMessage The new message to set.
+ * @return {module:client.Promise} Resolves: to nothing
+ * @return {module:http-api.MatrixError} Rejects: with an error response.
+ */
+MatrixClient.prototype.setStatusMessage = function(newMessage) {
+    return Promise.all(this.getRooms().map((room) => {
+        const isJoined = room.getMyMembership() === "join";
+        const looksLikeDm = room.getInvitedAndJoinedMemberCount() === 2;
+        if (isJoined && looksLikeDm) {
+            return this.sendStateEvent(room.roomId, "im.vector.user_status", {
+                status: newMessage,
+            }, this.getUserId());
+        } else {
+            return Promise.resolve();
+        }
+    }));
+};
+
 /**
  * @param {Object} opts Options to apply
  * @param {string} opts.presence One of "online", "offline" or "unavailable"