Refactor block retrieval

[BeritJanssen]

This PR implements the last steps to close #973.

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: backend/experiment/views.py

Function	Unhandled Issue
get_block	ValueError: Rules do not exist (anymore): VISUAL_MATCHING_PAIRS for block VMP (vmp) ... Event Count: 1
get_block	AttributeError: 'NoneType' object has no attribute 'section_set' /server/experime... Event Count: 1

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[drikusroor]

I made some minor comments, but I found it hard to review this PR as I don't really understand what its intention is

[drikusroor]

These properties already exist on ExtendedBlock so you can maybe remove them there (or here)

[BeritJanssen]

To clarify: the intention is that the get request to block doesn't return the next_round array anymore, but only information on theme etc., and most importantly, the session id. That id is then used to make a session/{id}/next_round request. I've always found the construct of having the request to block already contain actions defined in the rules files confusing and awkward to work with. That also does away with the necessity of having a distinction between Block / ExtendedBlock, as far as I could see.

[BeritJanssen]

Ah yes, but now I read back the changes I realize I also mixed in commits to remove the Consent option from the potential block actions.