console: use common/schema for dimensions

This is a bit less type-safe. We could keep type safety by redefining
all the consts in `query_consts.go` in `common/schema`, but this is
pointless as the goal is to have arbitrary dimensions at some point.

common/schema/root.go

@@ -36,4 +36,7 @@ type Column struct {
 	GenerateFrom  string
 	TransformFrom []Column
 	TransformTo   string
+
+	// For the console.
+	NotSelectable bool
 }