[iOS] Improve the appearance mechanism of the section completion checkmark #262

In the dropdown list of sections, after completing a section a blue checkmark is shown to the left of the section name.
In order for completion checkmark to appear, now after completing all units of the section user need to exit the course and enter it again and again go to the desired list of sections - as a result, the completion checkmark will appear.
Need to improve this behavior, that completion checkmark will appear immediatelly after completing all the units of the section, without re-entering in the course