frontend: speed-up the models.Batch related routes
    
    We don't have to do a SeqScan through all the Builds when we calculate
    the Batch tree — we can just go through those builds that really
    reference some batch.
    
    Also, constructing the Batch tree needs one SQL query for each node, and
    each of those queries took about ~200ms before — there was a missing
    index.
    
    Merges: #2091
    Fixes: #2090