When renaming a jobset, add a redirect from the old name

src/sql/hydra.sql

@@ -73,6 +73,16 @@ create table Jobsets (
 );
 
 
+create table JobsetRenames (
+    project       text not null,
+    from_         text not null,
+    to_           text not null,
+    primary key   (project, from_),
+    foreign key   (project) references Projects(name) on delete cascade on update cascade,
+    foreign key   (project, to_) references Jobsets(project, name) on delete cascade on update cascade
+);
+
+
 create table JobsetInputs (
     project       text not null,
     jobset        text not null,