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.

src/vector/platform/VectorBasePlatform.js

@@ -149,4 +149,12 @@ export default class VectorBasePlatform extends BasePlatform {
     getDefaultDeviceDisplayName(): string {
         return _t("Unknown device");
     }
+
+    /**
+     * Migrate account data from a previous origin
+     * Used only for the electron app
+     */
+    async migrateFromOldOrigin() {
+        return false;
+    }
 }