Special:MediaStatistics: data-sort-value also for number of files

The column "Number of files" is sortable, but does not contains plain
numbers, so it needs a data-sort-value to allow js to sort correct.

Change-Id: I9b7232b755dc8dab2cf04682103085273dada737
Merge "Special:MediaStatistics: data-sort-value also for number of files"