Added missing translation for back_to_shipping_categories_list

Fixes #4993
Do not blindly accept just any month for CreditCard expiry

Fixes #4971

Conflicts:
	core/app/models/spree/credit_card.rb
Silence two warnings

Fixes #4985

Silences this:

    [WARNING] You are not setting Devise.secret_key within your application!
    You must set this in config/initializers/devise.rb. Here's an example:

And this:

    [deprecated] I18n.enforce_available_locales will default to true in the future. If you really want to skip validation of your locale you can set I18n.enforce_available_locales = false to avoid this message.

in the sandbox app.