Add tool to migrate logins between origins

App checks at startup for an existing session, if there isn't one,
it will start the tool to check for a login in the file:// origin.
If there is one, it will copy the login over to the vector://vector
origin.

In principle this could also be used to migrate logins between
other origins on the web if this were ever required.

This includes a minified copy of the browserified js-sdk with
a getAllEndToEndSessions() function added to the crypto store
(https://github.com/matrix-org/matrix-js-sdk/pull/812). This is
not great, but for a short-lived tool this seems better than
introducing more entry points into webpack only used for the
electron app.

electron_app/src/originMigrator.js

@@ -0,0 +1,62 @@
+/*
+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.
+*/
+
+const { BrowserWindow, ipcMain } = require('electron');
+const path = require('path');
+
+async function migrateFromOldOrigin() {
+    console.log("Attempting to migrate data between origins");
+
+    // We can use the same preload script: we just need ipcRenderer exposed
+    const preloadScript = path.normalize(`${__dirname}/preload.js`);
+    await new Promise(resolve => {
+        const migrateWindow = new BrowserWindow({
+            show: false,
+            webPreferences: {
+                preload: preloadScript,
+                nodeIntegration: false,
+                sandbox: true,
+                enableRemoteModule: false,
+                webgl: false,
+            },
+        });
+        ipcMain.on('origin_migration_complete', (e, success, sentSummary, storedSummary) => {
+            if (success) {
+                console.log("Origin migration completed successfully!");
+            } else {
+                console.error("Origin migration failed!");
+            }
+            console.error("Data sent", sentSummary);
+            console.error("Data stored", storedSummary);
+            migrateWindow.close();
+            resolve();
+        });
+        ipcMain.on('origin_migration_nodata', (e) => {
+            console.log("No session to migrate from old origin");
+            migrateWindow.close();
+            resolve();
+        });
+        // Normalise the path because in the distribution, __dirname will be inside the
+        // electron asar.
+        const sourcePagePath = path.normalize(__dirname + '/../../origin_migrator/source.html');
+        console.log("Loading path: " + sourcePagePath);
+        migrateWindow.loadURL('file://' + sourcePagePath);
+    });
+}
+
+module.exports = {
+    migrateFromOldOrigin,
+};