Refactor register summary step

I noticed that the having the submit-functionality in the summary step in the registration process made things more complex than if it was in the top-level component of the wizard, so I moved it up there.

Also fixed the back-button which was broken.

frontend/src/routes/index.tsx

@@ -8,7 +8,7 @@ import fetchIntercept from 'fetch-intercept'
 import { useUserContext } from 'contexts'
 
 import Sponsor from 'routes/sponsor'
-import Register from 'routes/register'
+import Register from 'routes/sponsor/register'
 import FrontPage from 'routes/frontpage'
 import Invite from 'routes/invite'
 import InviteLink from 'routes/invitelink'