Don't have solidus_admin as a solidus dependency

For now it should stay opt-in.

core/lib/generators/solidus/install/install_generator.rb

@@ -34,6 +34,7 @@ class InstallGenerator < Rails::Generators::AppBase
     class_option :seed, type: :boolean, default: true, banner: 'Load seed data (migrations must be run)'
     class_option :sample, type: :boolean, default: true, banner: 'Load sample data (migrations and seeds must be run)'
     class_option :active_storage, type: :boolean, default: true, banner: 'Install ActiveStorage as image attachments handler for products and taxons'
+    class_option :admin_preview, type: :boolean, default: true, desc: 'Install the admin preview'
     class_option :auto_accept, type: :boolean
     class_option :user_class, type: :string
     class_option :admin_email, type: :string
@@ -159,6 +160,10 @@ def run_migrations
     end
 
     def install_solidus_admin
+      return unless options[:admin_preview]
+
+      say_status :installing, "SolidusAdmin", :blue
+      bundle_command 'add solidus_admin'
       generate 'solidus_admin:install'
     end
 

lib/solidus.rb

@@ -3,5 +3,4 @@
 require 'solidus_core'
 require 'solidus_api'
 require 'solidus_backend'
-require 'solidus_admin'
 require 'solidus_sample'

solidus.gemspec

@@ -21,7 +21,6 @@ Gem::Specification.new do |s|
   s.required_ruby_version = '>= 3.0.0'
   s.required_rubygems_version = '>= 1.8.23'
 
-  s.add_dependency 'solidus_admin'
   s.add_dependency 'solidus_api', s.version
   s.add_dependency 'solidus_backend', s.version
   s.add_dependency 'solidus_core', s.version