Invoice: add field on organization to retrieve all invoices

[PascalHelbig]

• [x] new table Invoice
  • [x] id column
  • [x] organizationId column
  • [x] invoiceDate column
  • [x] customerName column
  • [x] customerAddress column
  • [x] createdByUserId column
• [ ] new table InvoiceItem
  • [x] id column
  • [x] invoiceId column
  • [x] taskId column
  • [x] start column
  • [x] end column
  • [x] duration column
  • [ ] hourlyRate column
• [ ] new GraphQL type Invoice
• [ ] new GraphQL type InvoiceItem
• [ ] create relation fields between GraphQL types Invoice and InvoiceItem
• [ ] new GraphQL field invoices on Organization type (only org admins should access it)