Ability to relink GitLab.com user to another customers portal account

Problem

A GitLab.com user can only be linked to one customersDot account at a time, see gitlab-org&10201. Occasionally, users wish to change the cDot account that their user is linked to, but if their desired user is already linked to a separate account, they will be presented with an error

This GitLab.com account has already been linked to another Customers Portal account

Proposal

Allow users to change the linked user to a GitLab.com user even if it's already linked to a separate account. Prompt as necessary to ensure this behavior is desired.

See also similar proposal in #11023

Result

Customers don't need to rely on support for assistance

Reported examples

1. https://gitlab.zendesk.com/agent/tickets/575856
2. https://gitlab.zendesk.com/agent/tickets/581101

Support Priority Score: (1, -, -, -, -, -, 3, -, 2, -, -) => 6