Implement Use Case Selection screen (#8984)

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

res/css/structures/_SplashPage.scss

@@ -0,0 +1,63 @@
+/*
+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.
+*/
+
+.mx_SplashPage {
+    position: relative;
+    height: 100%;
+
+    &::before {
+        content: "";
+        display: block;
+        position: absolute;
+        z-index: -1;
+        opacity: 0.6;
+        background-image:
+            radial-gradient(
+                53.85% 66.75% at 87.55% 0%,
+                hsla(250, 76%, 71%, 0.261) 0%,
+                hsla(250, 100%, 88%, 0) 100%
+            ),
+            radial-gradient(
+                41.93% 41.93% at 0% 0%,
+                hsla(222, 29%, 20%, 0.28) 0%,
+                hsla(250, 100%, 88%, 0) 100%
+            ),
+            radial-gradient(
+                100% 100% at 0% 0%,
+                hsla(250, 100%, 88%, 0.174) 0%,
+                hsla(0, 100%, 86%, 0) 100%
+            ),
+            radial-gradient(
+                106.35% 96.26% at 100% 0%,
+                hsla(250, 100%, 88%, 0.4) 0%,
+                hsla(167, 76%, 82%, 0) 100%
+            );
+        /* blur to reduce color banding issues due to alpha-blending multiple gradients */
+        filter: blur(8px);
+        inset: -9px;
+        mask:
+            /* mask to dither resulting combined gradient */
+            url("$(res)/img/noise.png"),
+            /* gradient to apply different amounts of dithering to different parts of the gradient */
+            linear-gradient(
+                to bottom,
+                /* 10% dithering at the top */
+                rgba(0, 0, 0, 0.9) 20%,
+                /* 80% dithering at the bottom */
+                rgba(0, 0, 0, 0.2) 100%
+            );
+    }
+}