Signup: resume signup flow in the desktop app

[fabianapsimoes]

In a browser, if you partially complete signup and leave the page, we'll try to resume signup when the user returns to /start.

On the desktop app, on the other hand, we just redirect the user back to the login screen (See Automattic/wp-desktop#84). @mjangda suggested we should have the same behavior in both the app and the browser.

cc @breezyskies for input from Cerulean.

Related to #3221.

[breezyskies]

Agree with the previous suggestions to mimic the browser behavior here.

[stale]

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