fix: commentable_ids should be list of ids rather then a comma separated string

[xitij2000]

This along with upgrading the openedx-forum package to v0.3.2 or above will fix the issue where in-context discussions are not filtered to the current unit.

note: this can be dropped when upgrading to teak

Testing instructions:

• Create a tutor Sumac environment .
• Create new discussion topics under different units.
• They should all show up when you navigate to any unit.
• Switch the sumac environment to this branch and upgrade the openedx-forum package to v0.3.2
• Check that the discussion topics in each unit correspond to the current unit.

private ref: https://tasks.opencraft.com/browse/BB-10387

[farhaanbukhsh]

👍

✅ I tested this on the sandbox

• Created a post in one unit
• Tested that it isn't visible in any other unit
  ✅ I read through the code
  ❌ I checked for accessibility issues
  ❌ Includes documentation