Namespace navigation tab translations so that they don't clash with translations such as en.spree.orders.show

Fixes #3133

Conflicts:
	core/CHANGELOG.md
Add changelog for Cache-Control
[API] Include per_page element in products/index responses
Rely on correct total_count in products_controller_spec
[api] variant responses now include the full description text from the product
[api] Return display_single_amount and display_total_amount in LineItems responses

This is useful if the API client wants to display a well-formatted version of the price
Remove overridden deprecated methods and fix deprecation warning format thrown by them

Fixes #3055
Rename shipping_methods_zones table to spree_shipping_methods_zones

Fixes #3174
Correct habtm association for shipping_method zones

Related to #3174
[api] update variants controller spec to include 'description' in list of attributes
[api] StockMovements can never be altered (updated or destroyed), only created
Add translation for 'currently_unavailable' to dash translations
Fix documentation regarding in_taxon method

Fixes #3154
Sleep inside orders/shipments_spec to also wait for an API response
add `touch: true` option to Spree::Asset#viewable association

Fixes #3190
Fixing checkout unable to find CouponApplicator class when using threadsafe mode

NameError: uninitialized constant Spree::Core::CouponApplicator was thrown on checkout. Noticed coupon applicator was not being loaded in library.

Fixes #3177
Remove 'Overview' link check from backend now that dash has been removed