Add possibility for hosting provider IFrame

Add config option hosting_signup_iframe that
will render an action in the UserMenu and ProfileSettings
for setting up a host with some hosting provider
that provides an HTML to do that.

Add iframe for hosting provider signup content

src/components/structures/HostingProviderDialog.tsx

@@ -0,0 +1,33 @@
+/*
+Copyright 2020 Element
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import * as React from "react";
+import SdkConfig from "../../SdkConfig";
+
+interface IProps {}
+
+interface IState {}
+
+export default class HostingProviderDialog extends React.PureComponent<IProps, IState> {
+    public render(): React.ReactNode {
+        const hostingSignupUrl = SdkConfig.get().hosting_signup_iframe;
+        return (
+            <div className="mx_HostingProviderDialog_container">
+                <iframe src={hostingSignupUrl} />
+            </div>
+        );
+    }
+}