Add undo/redo implementation.

[proofit404]

Implementation:

1. There are two stories: purchase product and cancel subscription.
2. There is one data action class with do and undo methods: order repository.

Benefits:

1. Data access logic in one place. Easy to change do/undo together.
2. Two stories describes two separate processes independent from each other. Cancel subscription can have a different number of steps than purchase.