Properly handle GitHub username changes

[netomi]

A user can change his / her GitHub username. openvsx uses the login_name as primary key and will thus create a new user account after a change, and the user will not be associated with the previous GH username.

We already keep track of the unique GH userid which does not change after a rename, and should use that rather the login_name for association.

[netomi]

There are currently 258 occurrences of users that share the same authId which will indicate a login_name change:

SELECT 
    provider,
    auth_id,
    COUNT(*) AS occurrences,
	MAX(id)
FROM user_data
GROUP BY provider, auth_id
HAVING COUNT(*) > 1 and provider = 'github'
ORDER BY occurrences DESC;