Add property "amount" for Spends

[ShoshinNikita]

• Default value can be NULL or 0 (no value). Another option is to use 1 by default.
• Value can be rational: 0.152, 3, 1.75 and etc. The maximum precision is 0.0001. We can store it as v*1000 (like money)
• We can display price (use different name?) next to the cost. Price would be cost / amount. If amount is undefined, price would be -
• We have to display a lot of data on Month page. So, we can show price only on Search & Statistics page