Speed up the listing of jobset evals by caching some info in the DB

This denormalises the database, but it's the best we can do until
PostgreSQL has materialised views.

src/sql/upgrade-7.sql

@@ -0,0 +1,7 @@
+alter table JobsetEvals
+    add column nrBuilds integer,
+    add column nrSucceeded integer;
+
+update JobsetEvals e set
+    nrBuilds = (select count(*) from JobsetEvalMembers m where e.id = m.eval)
+    where hasNewBuilds = 1;