Stop polling when checking task lists on an issue

What does this MR do and why?

This fixes a race condition when something like this happens:

1. Polling request starts (DB query returns unchecked task)
2. User checks a task
3. Polling returns the stale response which reverses the user's action
4. User tries to check the task again. This now fails with a 409 because it's sending a "check" request for a task list that's already checked on the backend.

This is easier to understand with the screen recordings below.

Screenshots or screen recordings

Before	After
Screen_Recording_2021-10-20_at_12.11.23_AM	Screen_Recording_2021-10-20_at_12.08.22_AM

• Before this MR, you can also see the description flashing multiple times as realtime_changes updates the description.

• This is easier to reproduce with this patch:

  diff --git a/app/controllers/concerns/issuable_actions.rb b/app/controllers/concerns/issuable_actions.rb
  index e1e662a1968..876619277be 100644
  --- a/app/controllers/concerns/issuable_actions.rb
  +++ b/app/controllers/concerns/issuable_actions.rb
  @@ -86,6 +86,8 @@ def realtime_changes
         response[:updated_by_path] = user_path(issuable.last_edited_by)
       end
  
  +    sleep 2
  +
       render json: response
     end

  But even without that change, it's not that hard to reproduce.

• Tip to reproduce: Just check / uncheck the tasks sequentially until you notice the poller reversed your previous change. Then click on the task that was reversed to see the error banner.

• You can't test this on MRs because MRs don't poll for description changes.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.