Add in-app rebranding toasts & prompts

Either shows an informational dialog telling you the name has changed,
or a more naggy one if the user needs to log in on a different URL.
The new URL (if any) is hardcoded based on the current URL, and also
with a bonus config param in case other deployments need to do similar.

res/css/views/dialogs/_RebrandDialog.scss

@@ -0,0 +1,63 @@
+/*
+Copyright 2020 The Matrix.org Foundation C.I.C.
+
+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_RebrandDialog {
+    text-align: center;
+
+    a:link,
+    a:hover,
+    a:visited {
+        @mixin mx_Dialog_link;
+    }
+
+    .mx_Dialog_buttons {
+        margin-top: 43px;
+        text-align: center;
+    }
+}
+
+.mx_RebrandDialog_body {
+    width: 550px;
+    margin-left: auto;
+    margin-right: auto;
+}
+
+.mx_RebrandDialog_logoContainer {
+    margin-top: 35px;
+    margin-bottom: 20px;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+}
+
+.mx_RebrandDialog_logo {
+    margin-left: 28px;
+    margin-right: 28px;
+    width: 64px;
+    height: 64px;
+}
+
+.mx_RebrandDialog_chevron:after {
+    content: '';
+    display: inline-block;
+    width: 24px;
+    height: 24px;
+    mask-position: center;
+    mask-size: contain;
+    mask-repeat: no-repeat;
+    background-color: $muted-fg-color;
+    mask-image: url('$(res)/img/feather-customised/chevron-right.svg');
+}