action_status calculated based on action_start_date and action_end_date

[magul]

Description:

We have to pairs of date: action_start_date, action_end_date and started_at, finished_at. It occurs, that actually nobody knows why we have them both, so we decided to remove started_at and finished_at and use the other pair to calculate action_status.

Current behaviour:

action_status determined by started_at and finished_at.

Expected behaviour:

started_at and finished_at is removed fro database, serializers and fromtend. action_status is calculated using action_start_date and action_end_date.