Add sign_in_path JSON endpoint to session controller

What does this MR do and why?

Add sign_in_path JSON endpoint to session controller

This new path will allow the sign-in Vue app to determine the appropriate sign in path for a given login value (username or email address).

• For FOSS, always nil, which means stay here.
• For SaaS the sign-in may redirect to another cell or in the future to an organization-specific sign-in page.
  • Returning nil means, the user exists in this database, show the password field.
  • Returning a sign-in path means the user doesn't exist in this database, look elsewhere.
    • In later iterations, if the email domain is verified it will return a sign-in path to the organization, even if that exists on the local database.

References

• Add organization_sign_in feature flag (#574989)
• Session controller endpoint to determine sign i... (#572836)

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