Social Sign Up: Offer to login if account already exists

### Details

On https://wordpress.com/start/user, if a user tries to sign up with a social account that is already registered in WP.com, instead of being redirected to `/home`, they're sent to `/start/domains`.

That's fine if they don't have any site. The problem is that the session cookie is not set, so if they close that page and open https://wordpress.com/, they're shown the logged-out landing page again.

### Done is

- [ ] When the user signs in with "Continue with Google" from the signup flow **and already has a social WordPress.com account**, then set the login cookie right away (instead of at the end of the signup flow, which is what happens for a new user).
- [ ] User is redirected to `/home` instead of `/start/domains` if they already have a site
- [ ] When the user successfully signs in through the signup flow, dispatch a new event.

### Related

p1657729373359479/1657702289.138929-slack-C0347E545HR.

Start at the signup flow entry point at /start/user and trace the "Continue with Google" path for accounts that already exist. Check the session-cookie behavior and redirects to /home or /start/domains, then verify the three "Done is" conditions, including dispatching a new event after successful sign-in.

Written by the indexing model from the issue text.