[10.3] Prevent login with disabled OAuth providers

See merge request gitlab/gitlabhq!2296

(cherry picked from commit 4936650427ffc88e6ee927aedbb2c724d24b094c)

a0f9d222 Prevents login with disabled OAuth providers

The error saving the user is logged to application.log. Previously,
the entry had no context and was unusable - 'Error saving user:
[Email address already taken]'. Adding the auth hash UID and email
makes the error more helpful.

Fixed privilege escalation issue where manually set external users would be reverted back to internal users if they logged in via OAuth and that provider was not in the `external_providers` list.

This reverts commit c04e22fb, reversing
changes made to 0feab326.

some OAuth providers (kerberos for example) only provide a username and an email, but no name. Therefore
a signup fails because the name is empty. Best guess for the name is
probably the username, therefore use it as name.