Fix a few things with cancelling recovery reminder

* Put a cancel button on the first page of the create key backup
   dialog
 * Move settings logic into the room reminder: I know this is moving
   logic *into* a view but RoomView is quite heavyweight as it is.
 * Give the recovery reminder an explicit 'onDontAskAgainSet' rather
   than onFinished which was getting called with false when the last
   screen of the dialog was closed with the cancel 'x' rather than
   the 'close' button.

https://github.com/vector-im/riot-web/issues/8066

src/async-components/views/dialogs/keybackup/CreateKeyBackupDialog.js

@@ -1,5 +1,5 @@
 /*
-Copyright 2018 New Vector Ltd
+Copyright 2018, 2019 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.
@@ -503,7 +503,7 @@ export default React.createClass({
             <BaseDialog className='mx_CreateKeyBackupDialog'
                 onFinished={this.props.onFinished}
                 title={this._titleForPhase(this.state.phase)}
-                hasCancel={[PHASE_DONE].includes(this.state.phase)}
+                hasCancel={[PHASE_PASSPHRASE, PHASE_DONE].includes(this.state.phase)}
             >
             <div>
                 {content}