Use `unscoped` when create stock items in migration

By the time the migration runs the default scope with deleted_at was
already defined so mocking acts_as_paranoid doesn't make any difference

related to #3805
Ensure that preference keys are not prefixed with '/' if ENV['RAILS_CACHE_KEY'] is not set

Fixes #3831
remove extra include for Search module in Spree::BaseController

Fixes #3852
[frontend] remove redundant via in routes

Fixes #3853
Add changelogs for 2.1.2
[api] Add assurances that line item prices cannot be set while creating or updating orders
Add note about #3812 to core's CHANGELOG
centering search field on select2 plugin in admin area

Fixes #3854
Fixes #3842
Fix format of price input in products/new.html.erb
Use asset pipeline for jstree

Hard-coding links to /assets/ without fingerprints does not work on Rails 4
Remove version number from font awesome

The version number in the query string is not necessary for fingerprinted assets.

I'm not sure about the purpose of the #iefix anchor so I've left it in.

Additionally, some web servers do not handle query strings very well, such as SimpleHTTPServer.
[frontend] Stop people encountering uninitialized Taxon errors when they override ProductsController#show
Non-GET requests that would usually non-ssl-redirect will now respond with HTTP status 426

Conflicts:
	core/spec/lib/spree/core/controller_helpers/ssl_spec.rb
Include exception message from Spree::Core::GatewayErrors

This makes the error messages when payments fail slightly less vague than they used to be.

Fixes #3851
Bump activemerchant to 1.39.2
Clean up checks to avoid db connection on precompile

We still need to deal with the Preference / Config issue as stated in #3833
Bump awesome_nested_set

Get fix to avoid touching the database on assets precompile
Accept source attributes in PaymentsController to fix backend payment processing.

Fixes #3823
[backend] Use the controller name rather than Object when a model does not exist for authorize_admin method

Fixes #3622
Move preference_resuce.rb into Core, where it should've been

Fixes #3860