fix: ensure iframe visibility tracking is triggered on load

[Agrendalath]

The previous implementation (#1491) had a race condition that sometimes prevented XBlocks from being marked as viewed. Users had to scroll or resize the window to trigger visibility tracking instead of having it happen once the content loads.

Description

Testing instructions:

1. (Optional) Set COMPLETION_BY_VIEWING_DELAY_MS = 1000 in lms/envs/private.py for easier testing. The default value is 5000.
2. (Optional) Change publish_completion to publish_completions in lms/static/completion/js/CompletionOnViewService.js for easier testing. Otherwise, you will need to create a new XBlock for the next check or manually remove the BlockCompletion object from the DB to get the API call re-triggered the next time you view it.
3. Use Chrome for the next steps (do not check out this branch yet).
4. Create an HTML block with a single line. Check that the publish_completion request is not called in the Learning MFE.
5. Check out this branch and repeat the previous step. Ensure that the request has been published.

Deadline

"None"

Other information

Private-ref: BB-9602

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.83%. Comparing base (e6f7588) to head (448bd20).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1777   +/-   ##
=======================================
  Coverage   90.83%   90.83%           
=======================================
  Files         344      344           
  Lines        5792     5794    +2     
  Branches     1372     1371    -1     
=======================================
+ Hits         5261     5263    +2     
  Misses        512      512           
  Partials       19       19           

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[itsjeyd]

@Agrendalath Are you waiting for an OpenCraft-internal review here or are the changes ready for upstream review?

[Agrendalath]

@itsjeyd, this one is ready for upstream review.