Fix for API call with token
    
    Recently on staging deployment of src.stg.fedoraproject.org I encountered
    issue with `AttributeError: 'User' object has no attribute
    'can_create'`. After some digging with help from pagure matrix channel
    and from @abompard I was able to found out that there is a fix for that
    on src.fedoraproject.org, but it's not documented anywhere. So I'm
    submitting this PR with the fix.
    
    Here is the reasoning behind it. The API doesn't use OIDC
    authentication, but instead using token, so the `flask.g.fas_user`
    doesn't contain `can_create` attribute as this one is added during OIDC
    authentication. This patch is checking if the attribute is present first.