Add wrapped to set group is_joinable (#634)

which is exposed in the summary API under `summary.profile.is_joinable`.

If true, anyone can join the group. If false, an invite is required.

src/base-apis.js

@@ -443,6 +443,20 @@ MatrixBaseApis.prototype.setGroupProfile = function(groupId, profile) {
     );
 };
 
+/**
+ * @param {string} groupId
+ * @param {boolean} isJoinable whether anyone can join the group or only those who
+ *                             are invited.
+ * @return {module:client.Promise} Resolves: Empty object
+ * @return {module:http-api.MatrixError} Rejects: with an error response.
+ */
+MatrixBaseApis.prototype.setGroupJoinable = function(groupId, isJoinable) {
+    const path = utils.encodeUri("/groups/$groupId/joinable", {$groupId: groupId});
+    return this._http.authedRequest(
+        undefined, "PUT", path, undefined, { joinable: isJoinable },
+    );
+};
+
 /**
  * @param {string} groupId
  * @return {module:client.Promise} Resolves: Group users list object