Revert "Remove now-unused SystemTypes table"

This reverts commit 71d020735b.
Unfortunately there are still some cases where we need to set Hydra's
concurrency separately.  (Ideally, Hydra would start *all* queued
builds in parallel and let Nix take care of everything...)

tests/query-all-tables.pl

@@ -7,7 +7,7 @@ my $db = Hydra::Model::DB->new;
 my @sources = $db->schema->sources;
 my $nrtables = scalar(@sources);
 
-use Test::Simple tests => 42;
+use Test::Simple tests => 43;
 
 foreach my $source (@sources) {
     my $title = "Basic select query for $source";