[Feature] Setting to: Allow only Internal Registration

[6543]

like ALLOW_ONLY_EXTERNAL_REGISTRATION in app.ini but the other way arount.

by default false if true a user can link to external Oauth2 profiders but has to have already an account.

proposal: ALLOW_ONLY_INTERNAL_REGISTRATION

idear came up in: https://codeberg.org/Codeberg/Community/issues/225#issuecomment-72252

[stale]

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