Add Previous/Next product

[haricot]

This adds the possibility of linking the previous product and the next product in the detail of a product.

The template product_figure.html had to be reviewed because if a product is assigned to two category page, product.get_absolute_URL returns to the last category assigned which may not match the current category. that's why I replaced {{ product.get_absolute_URL}} with {{ request.path }} {{ product.slug }}

when the page is collapsed, previous / next are not displayed by choice. A swipe would be better.

I'm not particularly fond of this feature but it's part of a request.

[jrief]

👍 nice feature, always wanted to implement something like this myself!

[haricot]

closed in flavor #797