frontend: add /pending/all/ stats
    
    While we still keep the old /pending/ page, this new gives us more
    detailed overview for the whole queue (what owners, chroots or projects
    filled the queue, numbers of background jobs).
    
    The new route works with just "pending" jobs (contrary to the
    'for_backend' route(s)) while — for the performance reasons — it only
    needs a limited set of object data (joinedload && load_only tricks are
    useable).  This is why this commit defines another data_type=overview.
    
    The new route is pretty expensive, especially for queue of size in tens
    of thousands tasks or more.  This is why the page is cached.