Fix creating new PR from the page listing the pull-requests
    
    In this page we do two queries:
    - Give me all the branch of interest in the fork of the user
    - Give me all the branch of interest in the main repo (if the user is
      an admin)
    
    That second request was broken because the `repo_admin` variable was no
    longer used (instead if we use the one in flask.g).