Build Admin Tailwind assets when generating sandbox

This removes the need to start the sandbox from the main Solidus
folder with `bin/dev`, allowing to use `bin/dev` from the sandbox
folder as well. Using `rails server` remains discouraged.

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

@@ -189,7 +189,7 @@ def install_solidus_admin
       unless File.read(app_path.join('Gemfile')).include?('solidus_admin')
         bundle_command 'add solidus_admin -v ">= 0.2"'
       end
-      generate 'solidus_admin:install'
+      generate "solidus_admin:install #{options[:build_admin_tailwind] ? '--tailwind' : ''}"
     end
 
     def populate_seed_data