[frontend] Load taxon if params[:taxon] exists in ProductsController

Fixes a small bug where searching *twice* for the same term results in the taxon 'scope' being forgotten
Speed up admin/shipping_methods/_form with local var

I saw [almost 200](http://en.wikipedia.org/wiki/List_of_sovereign_states) calls  to `Spree::Zone Exists (0.3ms)` and thought there must be a better way.

Here we put the `@shipping_method.zones` into a local variable so we can hit that instead of the db.

Notes:

* The `to_a` method is necessary to force Rails to evaluate the `zones` scope. Otherwise we still get hundreds of db calls.
* If we want to, we could always replace the array of `Zones` with an array of `ids` -- replacing `to_a` with `.pluck(:id)` and having the checkbox tag read `include?(zone.id)` instead of `include?(zone)`. However I'm not sure this additional change is necessary.

Fixes #4535
No longer use insufficient_stock_lines in view

Fixes #4665
Fixes #4664
Stock::Quantifier must take a variant, not an id

@brchristian pointed out that the previous resolve_variant_id logic was
broken when a variant had been deleted.

This removes the resolve_variant_id private method from Quantifier, and
reuqires that a variant object be passed in. Finding associated variants
should be the resposibility of the caller, and the code to load a
variant unscoped need not be here.

Fixes #4660