OAuth-created Accounts cannot be unlinked

[coolaj86]

While reviewing the code I found that OAuth-created accounts are treated differently than oauth-linked accounts.

Currently under Manage account links (/user/settings/security) they cannot be unlinked from the oauth provider that created them, despite having their own username and email address.

I believe this is the desired behavior when ALLOW_ONLY_EXTERNAL_REGISTRATION is set, but should probably not be the behavior generally.

I'll include the fix in with another related PR, but I wanted to document it separately here for now.

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