Passphrase based key backups

Add passphrase support to backup creation stage

res/css/views/dialogs/keybackup/_CreateKeyBackupDialog.scss

@@ -0,0 +1,39 @@
+/*
+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_primaryContainer {
+    /*FIXME: plinth colour in new theme(s). background-color: $accent-color;*/
+    padding: 20px
+}
+
+.mx_CreateKeyBackupDialog_passPhraseInput {
+    width: 300px;
+    border: 1px solid $accent-color;
+    border-radius: 5px;
+    padding: 10px;
+}
+
+.mx_CreateKeyBackupDialog_passPhraseMatch {
+    float: right;
+}
+
+.mx_CreateKeyBackupDialog_recoveryKeyButtons {
+    float: right;
+}
+
+.mx_CreateKeyBackupDialog_recoveryKey {
+    width: 300px;
+}