feat: redirect to custom URL when third-party auth account is unlinked

[ArturGaspar]

Description

Allow redirecting to a custom URL when signing in via third-party auth when the account is not linked.

Testing instructions

TODO

Deadline

None

Other information

Alternate implementation of openedx/frontend-app-authn#1078

This does not have the problem where the login page briefly shows until third-party auth context is loaded, but has one big flaw which is that the user is still able to go to http://localhost:1999/login and see the error message.

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