Make kind of import configurable

[lukasrad02]

Exercise exports of the software can contain the exercise state at time of export ("currentState"), the initial state and list of actions to apply on it ("history"), or both. At the moment, using the history is preferred over using the currentState.

As we've decided that breaking backwards compatibility of actions that will only be applied in a running exercise is not a breaking change (i.e., it must be possible to import a scenario that was prepared but never started, but it must not be possible to import a scenario that has already been started), we should enable users to decide what kind of import they would like to use if their export contains a current state as well as a history.

New options should be:

• Use the currentState even if the export contains a history
• Use the history, but stop applying actions at the first [Exercise] Start action (so an already played exercise can still be used as "template", even if importing the full exercise and then creating a snapshot from the replay screen is not possible due to broken backwards compatibility of an action in the running exercise)

The current import button on the landing page should behave as before, but should have a dropdown to select another option (like the "Create PR"/"Create Draft PR" button on GitHub) to select one of the other options listed above. If an import fails, there should be a suggestion to use one of the other options with an explanation what will be imported and what will be lost when selecting this option.