[SPIKE] Replace Poxa with Action Cable

[sammo1235]

📝 A short description of the changes

• A very rough draft
• Includes basic behaviour:
• Two users cannot edit the same form at the same time
• If one user moves on to a new section (step) of the form, then then previous section is released for editing for the next user in the queue
• blocks a user from editing when they have two tabs of the same form opened.

I haven't fully worked out all the expected behaviours of the exiting implementation, so this may not exactly match what is needed.

🔗 Link to the relevant story (or stories)

Deployment implications

• Remove Pusher ENV vars and setup code.
• Allow action cable to talk to Redis (setup should be straightforward)

✅ Checklist

• Features that cannot go live are behind a feature flag/env var or specify deploy date and open PR as draft
• I have checked that commit messages make sense and explain the reasoning for each change
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have squashed any unnecessary or part-finished commits

🖼️ Screenshots (if appropriate - no PII/Prod data):

[sammo1235]

[TODO] remove once testing is complete - this output will muddy the waters