Builds: add a nullable jobset_id foreign key to Jobsets.

Also, adds an explicitly named "builds" accessor to the Jobsets
Schema object, which uses the project/jobset name.

src/sql/upgrade-61.sql

@@ -0,0 +1,10 @@
+-- Add the jobset_id columns to the Builds table. This will go
+-- quickly, since the field is nullable. Note this is just part one of
+-- this migration. Future steps involve a piecemeal backfilling, and
+-- then making the column non-null.
+
+ALTER TABLE Builds
+  ADD COLUMN jobset_id integer NULL,
+  ADD FOREIGN KEY (jobset_id)
+      REFERENCES Jobsets(id)
+      ON DELETE CASCADE;