refactor: remove existing user check from inquiry auth flow

[mc-jones]

The type of this PR is: Refactor

This PR solves PHIRE-670

Description

This PR removes the existing user check from the inquiry flow. See the parent of the ticket above for more detailed reasoning.

In slack, redirecting to the canonical auth flow was discussed as an option. I decided against this for the following reasons:

1. Scope. Looking at the code I felt this would require a much more significant refactor. The goal of this PR was to remove the existing user check from this inquiry flow and I wanted to stick as close to that as possible. However, I did try to make reasonable improvements to the current flow; such as, easier navigation between auth views and updating the error handling to be more like our main Auth flow.
2. In discussing the work with @jpoczik, I understand that we are considering a more general auth flow refactor. It felt reasonable to avoid integrating the canonical auth flow into the inquiry flow if the work will be redone in the medium term.

Considerations

• This work only affects the Auth portion of the inquiry flow. The post Auth steps (e.g. setting up a collector profile) have not been changed.
• I ran this by the PMs of Diamond (Auth) and Amber (Inquiry) to make sure they are comfortable with these changes from a product perspective.

old	new

[mc-jones]

https://github.com/artsy/gravity/pull/17445#issuecomment-1981320948

[dzucconi]

This all looks very reasonable. I appreciate the detail and the clear before/after screenshots.

[mc-jones]

Assigning this PR to myself. Realized I need to push integrity changes. Will sequence the rollout once those are ready.