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.
Adding a config option to set a paginate_by for products
Changing OSCAR_PRODUCTS_PAGINATE_BY to OSCAR_PRODUCTS_PER_PAGE
Prevent handling signals when loading fixtures
Remove CSS compilation from sandbox update script
Improve AbstractBasket lines cache handling

Currently, merge() only invalidated the line cache of the own instance,
and the public-facing merge_line() did not invalidate the cache at all.

Now, merge_line() invalidates the instances' line cache, and merge()
invalidates both instances' line cache.

I discovered this when using merge_line() to split a basket into
subbaskets and .is_empty would not reflect the current state.
Add display name to partner model

This allows a version of the partner to be shown to the customer and
avoids the tight coupling with the wrappers which use the name as a
lookup.

Fixes #354
Templates: Remove stray tag.
Add code field to partner model

This allows partners to be identified without using a name that might
change or be translated.

Unfortunately, this involves a few tricky migrations to move data around
in the correct order.

Fixes #434 but reverts work done for #354
Merge pull request #529 from maikhoepfel/fix/straytag

Templates: Remove stray tag.
Merge pull request #528 from maikhoepfel/fix/shippingsideeffect

Remove unintended side-effect
Merge pull request #526 from maikhoepfel/fix/ordercache

Improve AbstractBasket lines cache handling
Merged pull request #507 from informa/feature/product_update
ProductForm: fix bug when hiding related_products

The recent changes to the ProductUpdateView introduced a bug causing it
to break when the related_products field is hidden from the ProductForm.

This commit checks if the field is present before setting the queryset for
the related_products.
I also refactored the queryset out into a seperate function on
ProductForm to make it overridable.
Use BrowsableProductManager for related products

Was standard manager.
Fix messed up markup on registration page
Restrict django to version <1.5 until compatability branch is merged
Upgrade to WebTest==2.0

This found a few mark-up bugs where form tags used the single-element
form ("<form ... />") which prevented the fields being found by webtest.
Print contributing guidelines when installing in 'develop' mode

Hopefully this is useful and not annoying.
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