WIP e2e key backups

Continues from Matthew's work: adds a feature flag & panel in
user settings to create a backup.

Can't restore a backup yet, nor even continue backing up to the same
backup after a refresh.

res/css/views/dialogs/_CreateKeyBackupDialog.scss

@@ -0,0 +1,25 @@
+/*
+Copyright 2018 New Vector Ltd
+
+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_CreateKeyBackupDialog {
+    padding-right: 40px;
+}
+
+.mx_CreateKeyBackupDialog_recoveryKey {
+    padding: 20px;
+    color: $info-plinth-fg-color;
+    background-color: $info-plinth-bg-color;
+}