Implement assignment import and clear of budget

postgres/queries/assignments.sql

@@ -0,0 +1,13 @@
+-- name: CreateAssignment :one
+INSERT INTO assignments (
+        date, amount, category_id
+) VALUES (
+        $1, $2, $3
+)
+RETURNING *;
+
+-- name: DeleteAllAssignments :execrows
+DELETE FROM assignments
+USING categories
+INNER JOIN category_groups ON categories.category_group_id = category_groups.id
+WHERE categories.id = assignments.category_id AND category_groups.budget_id = @budget_id;