warn on logout too

src/Lifecycle.js

@@ -22,6 +22,7 @@ import Notifier from './Notifier';
 import UserActivity from './UserActivity';
 import Presence from './Presence';
 import dis from './dispatcher';
+import Modal from './Modal';
 import DMRoomMap from './utils/DMRoomMap';
 
 /**
@@ -289,7 +290,20 @@ export function logout() {
         return;
     }
 
-    return MatrixClientPeg.get().logout().then(onLoggedOut,
+    var QuestionDialog = sdk.getComponent("dialogs.QuestionDialog");
+    Modal.createDialog(QuestionDialog, {
+        title: "Warning",
+        description:
+            <div>
+                For security, logging out will delete any end-to-end encryption keys from this browser,
+                making previous encrypted chat history unreadable if you log back in.
+                In future this <a href="https://github.com/vector-im/riot-web/issues/2108">will be improved</a>,
+                but for now be warned.
+            </div>,
+        button: "Continue",
+        onFinished: (confirmed) => {
+            if (confirmed) {
+                MatrixClientPeg.get().logout().then(onLoggedOut,
                     (err) => {
                         // Just throwing an error here is going to be very unhelpful
                         // if you're trying to log out because your server's down and
@@ -302,6 +316,9 @@ export function logout() {
                         onLoggedOut();
                     }
                 );
+            }
+        },
+    });
 }
 
 /**