Implement service worker offline page

Also, make sure it can be translated.

src/store/index.js

@@ -136,6 +136,33 @@ const store = createStore({
             this.$matrix.logout();
             commit('logout');
         },
+        migrate({ state }, migrationData) {
+            if (migrationData && migrationData.type) {
+              let storage = migrationData.type == "session" ? sessionStorage : localStorage;
+              if (migrationData.settings) {
+                try {
+                  const settings = JSON.parse(migrationData.settings);
+                  storage.setItem(STORE_KEY_SETTINGS, migrationData.settings);
+                  state.useLocalStorage = migrationData.type != "session";
+                } catch (error) {
+                  console.error("Failed to migrate settings", error);
+                }
+              }
+              if (migrationData.user) {
+                const user = JSON.parse(migrationData.user);
+                state.auth.user = user;
+                return this.$matrix.migrate(user).then(
+                  (user) => {
+                    return Promise.resolve(user);
+                  },
+                  (error) => {
+                    return Promise.reject(error);
+                  }
+                );
+              }
+            } 
+            return Promise.reject("Migration error");
+        }
     },
     getters: {
         storage: state => {