Decorate Spree::UsersController when already existing

This commit allows to decorate the class `Spree::UsersController`
when already present (ie. when it's already included in the gem
`solidus_frontend`) or to define the class from scratch when it's
not (ie. when using a version of Solidus where `solidus_frontend`
does not include it).

See solidusio/solidus#2695 for the PR that adds the account page
to Solidus Frontend.

Once all supported versions of Solidus include the account page,
we will be able to get rid of the class definition, together with
the ERB view at `lib/views/frontend/spree/users/show.html.erb`.

lib/decorators/frontend/controllers/spree/users_controller_decorator.rb

@@ -0,0 +1,124 @@
+# frozen_string_literal: true
+
+module Spree
+  module UsersControllerDecorator
+    def self.prepended(base)
+      base.prepend_before_action :authorize_actions, only: :new
+      base.prepend_before_action :load_object, only: [:show, :edit, :update]
+    end
+
+    def create
+      @user = Spree::User.new(user_params)
+      if @user.save
+
+        if current_order
+          session[:guest_token] = nil
+        end
+
+        redirect_back_or_default(root_url)
+      else
+        render :new
+      end
+    end
+
+    def update
+      if @user.update(user_params)
+        spree_current_user.reload
+
+        if params[:user][:password].present?
+          # this logic needed b/c devise wants to log us out after password changes
+          unless Spree::Auth::Config[:signout_after_password_change]
+            bypass_sign_in(@user)
+          end
+        end
+        redirect_to spree.account_url, notice: I18n.t('spree.account_updated')
+      else
+        render :edit
+      end
+    end
+
+    private
+
+    def user_params
+      params.require(:user).permit(Spree::PermittedAttributes.user_attributes | [:email])
+    end
+
+    def authorize_actions
+      authorize! params[:action].to_sym, Spree::User.new
+    end
+
+    # When using a version of Solidus that includes the account page, we just need to
+    # decorate the existing controller with further behavior from this extension.
+    #
+    # On the other hand, when using a version of Solidus that does not include the
+    # account page, we need to define the controller from scratch.
+    #
+    # Once we drop the support for all Solidus versions that don't include the
+    # account page, we can just leave the decorator and remove anything else, including
+    # the view file at `lib/views/frontend/spree/users/show.html.erb`.
+    if defined?(Spree::UsersController)
+      Spree::UsersController.prepend(self)
+    else
+      class Spree::UsersController < Spree::StoreController
+        skip_before_action :set_current_order, only: :show, raise: false
+        prepend_before_action :load_object, only: [:show, :edit, :update]
+        prepend_before_action :authorize_actions, only: :new
+
+        include Spree::Core::ControllerHelpers
+
+        def show
+          @orders = @user.orders.complete.order('completed_at desc')
+        end
+
+        def create
+          @user = Spree::User.new(user_params)
+          if @user.save
+
+            if current_order
+              session[:guest_token] = nil
+            end
+
+            redirect_back_or_default(root_url)
+          else
+            render :new
+          end
+        end
+
+        def update
+          if @user.update(user_params)
+            spree_current_user.reload
+
+            if params[:user][:password].present?
+              # this logic needed b/c devise wants to log us out after password changes
+              unless Spree::Auth::Config[:signout_after_password_change]
+                bypass_sign_in(@user)
+              end
+            end
+            redirect_to spree.account_url, notice: I18n.t('spree.account_updated')
+          else
+            render :edit
+          end
+        end
+
+        private
+
+        def user_params
+          params.require(:user).permit(Spree::PermittedAttributes.user_attributes | [:email])
+        end
+
+        def load_object
+          @user ||= Spree::User.find_by(id: spree_current_user&.id)
+          authorize! params[:action].to_sym, @user
+        end
+
+        def authorize_actions
+          authorize! params[:action].to_sym, Spree::User.new
+        end
+
+        def accurate_title
+          I18n.t('spree.my_account')
+        end
+      end
+    end
+  end
+end