Feature/add balance to account (#193)

* Add balance attribute and calculate total balance

* Update changelog

prisma/migrations/20210703194509_added_balance_to_account/migration.sql

@@ -0,0 +1,6 @@
+-- AlterEnum
+ALTER TYPE "AccountType" ADD VALUE 'CASH';
+
+-- AlterTable
+ALTER TABLE "Account" ADD COLUMN     "balance" DOUBLE PRECISION NOT NULL DEFAULT 0,
+ADD COLUMN     "currency" "Currency" NOT NULL DEFAULT E'USD';