Implement Use Case Selection screen (#8984)

* Introduce new splash page wrapper
* Introduce new use case selection screen

src/components/structures/SplashPage.tsx

@@ -0,0 +1,27 @@
+/*
+Copyright 2022 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.
+*/
+
+import classNames from "classnames";
+import React, { DetailedHTMLProps, HTMLAttributes, ReactNode } from "react";
+
+interface Props extends DetailedHTMLProps<HTMLAttributes<HTMLElement>, HTMLElement> {
+    className?: string;
+    children?: ReactNode;
+}
+
+export default function SplashPage({ children, className, ...other }: Props) {
+    const classes = classNames(className, "mx_SplashPage");
+    return <main {...other} className={classes}>
+        { children }
+    </main>;
+}