Make ProductSearchForm customisable.

Note: Please review before merging.

All forms in the oscar.dashboard.catalogue app are overridable via the
common get_classes pattern, apart from the ProductSearchForm which is
imported directly from oscar.dashboard.catalogue.forms.
I've grepped through the source code, but couldn't find any hints on
why that would be necessary. As ProductSearchForm is quite simple
(not a ModelForm, and just two fields) I deemed it a bug and updated the
code to allow customising like everywhere else.
The tests run through fine, and the sandbox site works as expected.