Support linking & unlinking to an existing account when using SSO

[matrixbot]

This issue has been migrated from #9441.

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Currently the username picker says "<username> is not available, please choose another." if one tries to enter an existing username. It could instead offer to link the existing local account and ask for its password.

[Neko-LoverDisc]

please please allow us to unlink SSO

[zmberber]

is there a technical challenge with doing this? it seems weird from a design standpoint that i can't remove or add identity provider logins.

[panaceya]

It's very strange and a pity that there are no answers