Show the user's 3PID invite after they've reloaded the page

This is a step towards https://github.com/vector-im/element-web/issues/13430

Since we've stored the invite, we can send the user to it once they reload the page or revisit Element. We currently only support one invite at a time, but this should be fine for most cases. 

We only do this restoration if the next screen isn't set to avoid breaking the user out of an expected flow.

As an added touch, this also ensures that the email address is pre-filled on the registration page if needed, just in case the user refreshes before getting to the submit button.

src/Lifecycle.js

@@ -42,6 +42,7 @@ import {Mjolnir} from "./mjolnir/Mjolnir";
 import DeviceListener from "./DeviceListener";
 import {Jitsi} from "./widgets/Jitsi";
 import {SSO_HOMESERVER_URL_KEY, SSO_ID_SERVER_URL_KEY} from "./BasePlatform";
+import ThreepidInviteStore from "./stores/ThreepidInviteStore";
 
 const HOMESERVER_URL_KEY = "mx_hs_url";
 const ID_SERVER_URL_KEY = "mx_is_url";
@@ -676,7 +677,17 @@ async function _clearStorage() {
     Analytics.disable();
 
     if (window.localStorage) {
+        // try to save any 3pid invites from being obliterated
+        const pendingInvites = ThreepidInviteStore.instance.getWireInvites();
+
         window.localStorage.clear();
+
+        // now restore those invites
+        pendingInvites.forEach(i => {
+            const roomId = i.roomId;
+            delete i.roomId; // delete to avoid confusing the store
+            ThreepidInviteStore.instance.storeInvite(roomId, i);
+        });
     }
 
     if (window.sessionStorage) {