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

[ArturGaspar]

Description

Same as #1078.

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

Supporting information

#1078

Testing instructions

See #1078.

Other information

Private-ref: https://tasks.opencraft.com/browse/BB-7742

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[ArturGaspar]

Wrong repository, sorry.