Discounts in % in invoice and delivery notes

closes #475
For Front end refer : https://gitlab.com/gnukhata/gkwebapp/merge_requests/1048
Added new field 'discflag' in invoice, invoicebin and delchal table which will show that the discount is in percent or in amount.
When discflag = 1 then it shows discount in amount.(this is default value)
when discflag = 16 then it shows discount in percent.
On the basis of flag further changes are done in invoice and delchal API.

Also resolved issue #478 (closed)
Added two columns in product table for discount in percent and discount in amount.
Modified product API.