Store aggregate members in the database

For presentation purposes, we need to know what builds are part of an
aggregate build.  So at evaluation time, look at the "members"
attribute, find the corresponding builds in the eval, and create a
mapping in the AggregateMembers table.

src/sql/upgrade-19.sql

@@ -0,0 +1,5 @@
+create table AggregateMembers (
+    aggregate     integer not null references Builds(id) on delete cascade,
+    member        integer not null references Builds(id) on delete cascade,
+    primary key   (aggregate, member)
+);