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.

src/components/structures/MatrixChat.js

@@ -48,6 +48,8 @@ import SettingsStore, {SettingLevel} from "../../settings/SettingsStore";
 import { startAnyRegistrationFlow } from "../../Registration.js";
 import { messageForSyncError } from '../../utils/ErrorUtils';
 
+import SuggestKeyRestoreHandler from "../../SuggestKeyRestoreHandler";
+
 /** constants for MatrixChat.state.view */
 const VIEWS = {
     // a special initial state which is only used at startup, while we are