Introduce a concept of "non-urgent" toasts

This is somewhat expected to be temporary.

res/css/structures/_NonUrgentToastContainer.scss

@@ -0,0 +1,35 @@
+/*
+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_NonUrgentToastContainer {
+    position: absolute;
+    bottom: 30px;
+    left: 28px;
+    z-index: 101; // same level as other toasts
+
+    .mx_NonUrgentToastContainer_toast {
+        padding: 10px 12px;
+        border-radius: 8px;
+        width: 320px;
+        font-size: $font-13px;
+        margin-top: 8px;
+
+        // We don't use variables on the colours because we want it to be the same
+        // in all themes.
+        background-color: #17191C;
+        color: #fff;
+    }
+}