Add support for computing averages for 444bf and 555bf

[jfly]

@jazzthief81 did this a few years ago for 333bf, see 746f586.

Since we now recognize 444bf and 555bf means on the WCA website, the workbook assistant should also be updated to do this.

[Baiqiang]

Maybe we can calculate the average when submitting results? This is part of the website so it doesn't depend on the WA.