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

res/css/views/dialogs/_HostingProviderDialog.scss

@@ -0,0 +1,44 @@
+/*
+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.
+*/
+
+.mx_HostingProviderDialog .mx_Dialog {
+    width: 60%;
+    height: 70%;
+    overflow: hidden;
+    padding: 0;
+    max-width: initial;
+    max-height: initial;
+    position: relative;
+}
+
+.mx_HostingProviderDialog_container {
+    position: absolute;
+    top: 0;
+    bottom: 0;
+    left: 0;
+    right: 0;
+
+    iframe {
+        width: 100%;
+        height: 100%;
+        border: none;
+        background-color: #fff;
+    }
+}
+
+.mx_HostingProviderTrigger {
+    cursor: pointer;
+}