Make Alchemy's signup page work with Solidus' user controller

Solidus ships an admin users controller within the core backend
distribution, while Alchemy does this within the auth extension
`alchemy-devise`. So when using `alchemy-devise` with `solidus-backend`,
Solidus` admin users controller takes over, and we need to make sure our
non-users has the right abilites to create a first admin user, and that
that admin user then has admin rights, too.

Author: mamhoff

app/patches/controllers/alchemy/solidus/spree_admin_users_controller_patch.rb

@@ -0,0 +1,24 @@
+# /home/anselm/code/alchemy-solidus/app/patches/controllers/alchemy/solidus/spree_admin_users_controller_patch.rb
+
+module Alchemy
+  module Solidus
+    module SpreeAdminUsersControllerPatch
+      def self.prepended(base)
+        base.after_action :assign_admin_roles_to_first_user, only: :create
+      end
+
+      private
+
+      def assign_admin_roles_to_first_user
+        if Spree.user_class.count == 1
+          user = Spree.user_class.last
+          user.spree_roles = [Spree::Role.find_or_create_by(name: "admin")]
+          user.alchemy_roles = ["admin"]
+          user.save
+        end
+      end
+    end
+  end
+end
+
+::Spree::Admin::UsersController.prepend(Alchemy::Solidus::SpreeAdminUsersControllerPatch)

app/patches/models/alchemy/solidus/alchemy_devise_ability_patch.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+module Alchemy
+  module Solidus
+    module AlchemyDeviseAbilityPatch
+      def initialize(user)
+        super
+
+        # Without these abilities, Alchemy's signup page does not work with Solidus' UsersController.
+        if Alchemy::User.count == 0
+          can :admin, Alchemy::User
+          can :update_password, Alchemy::User
+          can :update_email, Alchemy::User
+        end
+      end
+
+      if defined?(::Alchemy::Devise::Ability)
+        ::Alchemy::Devise::Ability.prepend self
+      end
+    end
+  end
+end

config/initializers/spree.rb

@@ -1,5 +1,6 @@
 if defined?(Alchemy::Devise::Engine)
   Spree.user_class = "Alchemy::User"
+  Spree::PermittedAttributes.user_attributes << :login
 end
 
 Rails.application.config.after_initialize do

lib/alchemy/solidus/engine.rb

@@ -14,6 +14,7 @@ module Alchemy
   module Solidus
     class Engine < ::Rails::Engine
       include SolidusSupport::EngineExtensions
+
       engine_name "alchemy_solidus"
 
       initializer "alchemy_solidus.assets", before: "alchemy.importmap" do |app|
@@ -30,6 +31,9 @@ class Engine < ::Rails::Engine
       config.to_prepare do
         Alchemy.register_ability ::Spree::Ability
         ::Spree::Ability.register_ability ::Alchemy::Permissions
+        Alchemy.registered_abilities.reject { _1 == Spree::Ability }.each do |ability|
+          ::Spree::Ability.register_ability ability
+        end
 
         if SolidusSupport.frontend_available?
           # Allows to render Alchemy content within Solidus' controller views