resultsdb_directive: change default log link to artifactsdir
    
    This changes the default log link visible in resultsdb from task stdio
    output to artifactsdir directory (which contains stdio, debug log and
    others). This only applies if the task hasn't set `artifact` in
    resultyaml (that overrides the default).
    
    The reasoning is that taskotron stdio has grown larger and larger and
    nowadays it's very hard to find relevant task output inside it anyways.
    Tasks really need to create specialized logs at least in case of failure
    (and all existing ones do). The new default is much better for
    a) people debugging a problem - because they have clear access to all
    available files
    b) tasks with multiple log files which don't want to set any of them as
    default (because it would make accessing the other ones much harder) -
    with the new solution they see all artifacts in a directory view
    
    Differential Revision: https://phab.qa.fedoraproject.org/D1210