Subscriptions: ask user to login after subscribing to a free tier

[zegl]

We used to have this, but it's gone now.

1. As a logged out user
2. Subscribe to the free tier via email
3. You're sent to the /subscribe takeover. Click "skip" or "subscribe" on the free tier.
4. You're sent back to the org page with no verification that you subscribed to anything.

I suggest that we re-add the "congratulations" modal that also asks the subscribing user to log in by "verifying their email".