API: switch to sortParam

[himdel]

Extracted from #4507

Our various APIs use ?sort, ?ordering or ?order_by for sorting.

Before:

BaseAPI just uses sort (and only deals with page & page_size -> offset & limit mapping),
PulpAPI adds a useOrdering = false; param, and overrides list to map sort to ordering depending on that,
PulpAPI subclasses set useOrdering = true; where applicable,
LegacyAPI adds a sortParam = 'order_by'; and overrides list to map sort to that,
LegacyAPI subclasses setting sortParam to either sort or order_by.

After:

BaseAPI sets sortParam = 'sort';, and maps sort to the value of sortParam in list,
PulpAPI sets sortParam = 'ordering';,
LegacyAPI sets sortParam = 'order_by';,

the exceptions being:

• collection version using order_by (not sort)
• EE namespaces using sort (not ordering)
• role namespaces using sort (not order_by)

The only real change should be that CollectionVersionSearchAPI now uses order_by.