add confirm-element to steps

[jonasjonas]

Add a new confirm element to steps, that would display all data that the user provided. (See #4)
The user could confirm, or go back to previous steps to change some values.

Before displaying the data, we would calculate the hash of all previous data and when the user presses submit, we compare the saved values with the former hash. This way, we can make sure, that the data has not changed or edited in another window, the the user confirms everything what was displayed.