PA-444 median applications per week

The calculation for median applications per week now excludes weeks
where there were zero applications. I understand that this will make
calculations slightly wrong for councils who tend to have more genuine
weeks with zero applications, but the tradeoff is with the inevitable
scrapers that fail for long periods at a time now returning numbers
other than 0.
Merge remote-tracking branch 'pserwylo/master'
Added small explanatory text for new calculation of media applications