Align asset filenames across all asset sources

While taking care of #3466 by moving custom stylesheets under vendor/

app/views/layouts/rails_admin/_head.html.erb

@@ -9,8 +9,8 @@
   <%= javascript_pack_tag "rails_admin", async: true %>
 <% when :webpack, :sprockets %>
   <% handle_asset_dependency_error do %>
-    <%= stylesheet_link_tag "rails_admin/application.css", media: :all %>
-    <%= javascript_include_tag "rails_admin/application.js", async: true %>
+    <%= stylesheet_link_tag "rails_admin.css", media: :all %>
+    <%= javascript_include_tag "rails_admin.js", async: true %>
   <% end %>
 <% else
      raise "Unknown asset_source: #{RailsAdmin::config.asset_source}"

lib/rails_admin/engine.rb

@@ -13,8 +13,8 @@ class Engine < Rails::Engine
     initializer 'RailsAdmin precompile hook', group: :all do |app|
       if app.config.respond_to?(:assets)
         app.config.assets.precompile += %w[
-          rails_admin/application.js
-          rails_admin/application.css
+          rails_admin.js
+          rails_admin.css
         ]
         app.config.assets.paths << RailsAdmin::Engine.root.join('src')
         require 'rails_admin/support/esmodule_preprocessor'