frontend: admin permission check in 'can_build_in()'
    
    Commit 8e8867b5a caused that we didn't compare the admin permissions of
    the logged user, but the owner of copr we tried to build against.
    
    Fix test-cases which didn't expect that the requesting user may be
    admin, and add new test-case for the bug itself (that normal user can
    not build in admin's project).
    
    Fixes: #970