Add model-based test for data entry

[jschuurk-kr]

Follow-up of PR #840: spike: model-based testing of data entry forms with xstate

In scope

• add model-based test from spike
• improve code: helper functions (e.g. check values of voters and votes fields), split up states and events in test into multiple objects, extend page objects
• write code that checks if states and actions from machine are present in test and vice-versa
  • Currently there's no check that the test contains code for both all states and all events. The tests will just run. In case of an event without code, they will very likely fail; in case of a state without code the test will just pass. For the states in the machine there is a check that each state is defined.
• decide what from "Not in scope" needs to be done and create issues for those tasks

Not in scope

• data entry that results in error
• data entry that results in warning
• closing save/discard modal