feat(accounts): Add redirect handling to /accounts/social SignupView

[denisemauldin]

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[denisemauldin]

@pennersr Not sure if tests are necessary, but happy to write some based off suggestions.

[pennersr]

When the form is valid, the social signup view ends with a call to: helpers.complete_social_signup(). Inside that method, the next URL stashed in the sociallogin is already used (sociallogin.get_redirect_url(request)), which is identical to the next = sociallogin.state.get(REDIRECT_FIELD_NAME, None) call that you are adding. So redirects should already be properly be handled. if not, then we definitely could use a test case to expose the problem you are trying to address here.