Add post-login complete security flow

This adds a step after login to complete security for your new session. At the
moment, the only verification method is entering your SSSS passphrase, but nicer
paths will be added soon.

This new step only appears when crypto is available and the account has
cross-signing enabled in SSSS.

Fixes https://github.com/vector-im/riot-web/issues/11214

res/css/structures/auth/_CompleteSecurity.scss

@@ -0,0 +1,51 @@
+/*
+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_CompleteSecurity_header {
+    display: flex;
+    align-items: center;
+}
+
+.mx_CompleteSecurity_headerIcon {
+    width: 24px;
+    height: 24px;
+    margin: 0 4px;
+    position: relative;
+}
+
+.mx_CompleteSecurity_heroIcon {
+    width: 128px;
+    height: 128px;
+    position: relative;
+    margin: 0 auto;
+}
+
+.mx_CompleteSecurity_body {
+    font-size: 15px;
+}
+
+.mx_CompleteSecurity_actionRow {
+    display: flex;
+    justify-content: flex-end;
+
+    .mx_AccessibleButton {
+        margin-inline-start: 18px;
+
+        &.warning {
+            color: $warning-color;
+        }
+    }
+}