Created invoices table

server/db/schema/invoices.sql

@@ -0,0 +1,9 @@
+CREATE TABLE invoices (
+  id INT PRIMARY KEY UNIQUE NOT NULL DEFAULT nextval('invoice_id_seq'),
+  event_id INT NOT NULL REFERENCES events(id),
+  start_date DATE NOT NULL,
+  end_date DATE NOT NULL,
+  is_sent BOOL NOT NULL DEFAULT False,
+  payment_status payment NOT NULL DEFAULT 'none'
+
+);