API: applyCellFilter_hook to override cellfilter

in some cases admin might want to speed-up rendering of filtered depot page
using env-specific assumptions. This change introduces applyCellFilter_hook
that allows to get rid of listCells() call when rendering depot page.

applyCellFilter: new wrapper function that takes $realm and $cellfilter,
	calls applyCellFilter_hook and then filterCellList(listCells())
   	unless hook returned list.
renderRackspace: use new applyCellFilter
renderRow: idem
renderDepot: idem
renderCellList: idem