OSS: Add a first-time logged-out developer login and signup flow

[nb]

In #436 we added an interstitial screen for logged out users in the development environment. The login and signup links however break the flow – after completion they don’t get us back to the local site.

Here’s how we can make the experience better:

• Create new signup flow, which doesn’t include plans and at the end brings us back to the dev environment.
• Make sure the login redirects back to the dev environment (might need server-side changes to allow it)