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/components/structures/MatrixChat.tsx

@@ -1214,6 +1214,14 @@ export default class MatrixChat extends React.PureComponent<IProps, IState> {
                     // the homepage.
                     dis.dispatch({action: 'view_home_page'});
                 }
+            } else if (ThreepidInviteStore.instance.pickBestInvite()) {
+                // The user has a 3pid invite pending - show them that
+                const threepidInvite = ThreepidInviteStore.instance.pickBestInvite();
+
+                // HACK: This is a pretty brutal way of threading the invite back through
+                // our systems, but it's the safest we have for now.
+                const params = ThreepidInviteStore.instance.translateToWireFormat(threepidInvite);
+                this.showScreen(`room/${threepidInvite.roomId}`, params)
             } else {
                 // The user has just logged in after registering,
                 // so show the homepage.
@@ -2015,12 +2023,13 @@ export default class MatrixChat extends React.PureComponent<IProps, IState> {
             view = <Welcome />;
         } else if (this.state.view === Views.REGISTER) {
             const Registration = sdk.getComponent('structures.auth.Registration');
+            const email = ThreepidInviteStore.instance.pickBestInvite()?.toEmail;
             view = (
                 <Registration
                     clientSecret={this.state.register_client_secret}
                     sessionId={this.state.register_session_id}
                     idSid={this.state.register_id_sid}
-                    email={this.props.startingFragmentQueryParams.email}
+                    email={email}
                     brand={this.props.config.brand}
                     makeRegistrationUrl={this.makeRegistrationUrl}
                     onLoggedIn={this.onRegisterFlowComplete}