Remove first_round from rules files

[BeritJanssen]

After #1132, consent will always be shown via the consent field tied to the Experiment parent.

Calling the endpoint block/{id} will do the following:

• check the participant
• start a session with the current / new participant and the current block, and return its id to the frontend

After that, the block will be handled through the session/{id}/next_round endpoint
Steps:
Backend

• remove Consent actions from all first_round methods
• move Playlist views to next_round (only for those rules where it's conceivable we may have multiple playlists tied to them)
• move every Explainer to next_round
• remove first_round methods
• migrate existing Consent forms to Experiment level if applicable

Frontend

• remove Consent as an option from Block

[drikusroor]

After #1132, consent will always be shown via the consent field tied to the Experiment parent.

I mean... technically the field is there, but do we have functionality that also checks if the participant has accepted the consent form before starting the experiment / block and shows the consent form if that's not the case? If not, perhaps we should make that before doing this refactor. 🤔