Add flake configuration to the web interface

src/sql/upgrade-57.sql

@@ -0,0 +1,6 @@
+alter table Jobsets alter column nixExprInput drop not null;
+alter table Jobsets alter column nixExprPath drop not null;
+alter table Jobsets add column type integer default 0;
+alter table Jobsets add column flake text;
+alter table Jobsets add check ((type = 0) = (nixExprInput is not null and nixExprPath is not null));
+alter table Jobsets add check ((type = 1) = (flake is not null));