Ci 2827 user error handling

[leannecornish-ft]

Describe your changes

1. We have found a bug that causes users that have previously had an FT account to comment with their old coral account. We will be adding an additional check to the auth endpoint in next-comments-api, which will return a 403 when an old coral account is found. This PR checks for this status and returns a user-friendly error to the client.

2. There's also some refactoring to make it more performant and easier to understand
3. There's a bug fix to close the modal properly as it was preventing it being reopened

Additional context

JIRA ticket	Figma design
Parent: CI-2823
Sub: CI-2827

Checklist before requesting a review

• I have applied percy label for o-[COMPONENT] or chromatic label for o3-[COMPONENT] on my PR before merging and after review. Find more details in CONTRIBUTING.md
• If it is a new feature, I have added thorough tests.
• I have updated relevant docs.
• I have updated relevant env variables in Doppler.