Fix grade conversion for old South African grading system

[Willemfvz]

As per the discussion here, the old South African grade conversion is currently incorrect on the site. The grading system is still being used in some areas. In the link above, Cormac suggested using a conversion table linked here:

In the chart the French grading vs SA grading is off by one compared to The Crag's grade conversion chart, but this probably does not need to change on the website. The focus of this issue is the chart's conversion from SA to Old SA, which is more accurate (especially at higher grades where the current website conversion is significantly off).