Remove 'hand-compiled' JavaScript nonsense from core

We are now going to rely on responses from API for this
Must require_self in spree_core.js

This is so Spree JS variable is defined before all other JavaScript files are loaded in the admin backend
Correct base_url usage in taxonomy.js
Revert "Remove unused Admin::PropertiesController"

This reverts commit 6ad38fd09d9b31d36f6594b001fe01b2c846456e.
Remove guard dependencies from test group
email_spec should only be a dependency of core
Remove core dependency from models

Because core depends on models, not tother way round
Remove ActiveResource dependency for Alert, use HTTParty instead
Move core controllers+views to frontend; move models to core
Reshuffle frontend and core

This is a combination of 27 commits.

* Move search base fron frontend to core
* Move Spree::Money from frontend to core
* move mailers to core
* Move mailer views to core
* Require core not models in frontend gemspec
* Fix require in product_filter_spec
* frontend stuff now lives at lib/spree/frontend
* Core engine is now once again for isolating the engine namespace
* Move user class extensions to core
* Move UserBanners to Core
* Frontend now prepends routes to core
* Rename Core references in Frontend to Frontend
* Fix spree_frontend gemspec
* Fix spree_core.gemspec
* Remove unnecessary scopes file
* Rename spree/core.rb inside frontend to spree/frontend.rb
* Fix ControllerHelpers reference in Spree::BaseController
* Fix require to action_callbacks withn Admin::ResourceController
* Fix module reference for ControllerHelpers inside Spree::StoreController
* Fix testing_support requires within spec_helper
* Fix requires in frontend.rb
* Moved ControllerHelpers back into Core.
* This is because API no longer depends on frontend
* Move SSL requirement into core from frontend
* Add require for cancan inside core.rb
* Add frontend to travis
* Add spree_core.rb
* Add frontend to Rakefile
Define money helper inside OrderMailer and ShipmentMailer, no longer depend on Spree::BaseHelper
Remove un-used Spree::Scopes::Dynamic module
Deprecate RedirectLegacyProductUrl middleware
Newline in user_banners.rb
Fix TestingSupport references inside API
Move rabl dependency to api
[api] Only deface view if Deface is defined
default return state for taxonomy/jstree response is actually closed, not open
Move 'custom products' shared context out of spec_helper and into completely separate file
Remove excessive respond_with calls from Admin::LineItemsController