Add button to purge rooms to room list (#44)

* Add button to purge rooms to room list

Add button (BulkDeleteButton) to delete rooms with no local users:
- purge rooms `POST /_synapse/admin/v1/purge_room`
(https://github.com/matrix-org/synapse/blob/master/docs/admin_api/purge_room.md)

* Bugfix of merge 'master'

* Change from purge room to delete room endpoint

src/components/rooms.js

@@ -1,4 +1,4 @@
-import React from "react";
+import React, { Fragment } from "react";
 import { connect } from "react-redux";
 import {
   BooleanField,