Implement accounting system

[nspo]

• Introduce ability to generate periodical accounting statements
• E.g. Accounting Statement 2017 includes and locks all invoices from Jan 1st to Dec 31st
• Admins see sum over all cash and bank payments as well as "other" payments
• Payments and purchases can have "accounting groups" - e.g. all coffee purchases and payments b/c of new coffee belong to one accounting group - if sum > 0, a profit is made by offering coffee