[BB-3825] Fix: Hide new post indicator on thread open
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Fixes the removal of the new post indication in unit discussion once a thread has been opened.
When an unread thread is clicked, the user expects the blue margin highlight to be removed. However, it is not removed even after opening the thread a number of times. This behaviour is different from that shown in the Discussion tab where the margin highlight is removed. See below
Login to the LMS with account A
Open a unit with a discussion block. Make a few contributions to the discussion by posting a few comments
Logout and then login with a different account (account B) that is enrolled to the same course.
Visit the same unit. You should be able to see the posts made by account A highlighted with a blue left border.
Click on one post to open the thread. This should open a view of the thread.
Click on All Posts to go back to the list of posts. Confirm that the highlight on this thread has been removed.
Open other threads to confirm this fix.