MM-35805: following a thread should mark it as read (#17717)

Currently upon following a thread the thread is unread,
this commit changes that by explicitly marking it as read.

api4/user_test.go

@@ -5746,6 +5746,7 @@ func TestFollowThreads(t *testing.T) {
 		})
 		CheckNoError(t, resp)
 		require.Len(t, uss.Threads, 1)
+		require.GreaterOrEqual(t, uss.Threads[0].LastViewedAt, uss.Threads[0].LastReplyAt)
 
 	})
 }