Reorganize first_round logic

[BeritJanssen]

Right now, the rules' first_round method will trigger the frontend to:

• to show a Playlist view
• show a Consent view
• show an Explainer

At the time of writing, the Explainer must be the last element in the first_round array, and the array cannot be empty.

After #928, the method will only trigger the frontend to:

• show a Playlist view
• show an Explainer

(Consent will be coupled to an Experiment, not to a Rule anymore.)
The first_round (or, perhaps "setup"?) method and its corresponding frontend logic should then be able to do the following:

• show different combinations of one or more Explainers and an optional Playlist view (order should not matter)
• show nothing at all, proceed immediately to next_round