fix: do not query user_link for deleted accounts

[Emyrk]

Closes #10972

What this does

This solution is a patch, I think we need to create better unique constraints on the database to prevent other types of bugs in the future.

If you change your github email and log in, we link based on your Github UID. So your email is updated on Coder. This was failing if you previously deleted an account that was linked to the same github. The previous GetUserLinkByLinkedID would fetch a deleted user, and hit this line:

coder/coderd/userauth.go

Lines 1728 to 1729 in 4f0203d

	// If the user was deleted, act as if no account link exists.
	user = database.User{}

This would then instead make a new account. A simple fix is to ignore deleted acounts.

Future work

• It is possible to have more than 1 user_link per account. This does exist on prod, and is a mistake imo.
• It is possible to have multiple user_links with the same linked_id because of deleted users. I am not sure what the solution is here 🤔. When we delete an account, should we do something to the user_link?

[Emyrk]

• main
  • chore: add database test fixture to insert non-unique linked_ids #12111
    • fix: do not query user_link for deleted accounts #12112 👈
      • fix: add postgres triggers to remove deleted users from user_links #12117

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[johnstcn]

I think this patch is OK for now, but we should also:

1. Delete user_links when a user is deleted
2. Update the schema to ensure that this situation cannot occur in future.

We should create issues for the above change(s) and reference them in the unit test so we do not forget to do them.

Also: the PR title is incorrect as this is no longer just a chore but actually changes functionality.