DEV: Use default admin routes for plugins with settings (#30941)

This change adds a sidebar link for each plugin that fulfils the following criteria:

- Does not have an explicit admin route defined in the plugin.
- Has at least one site setting (not including enabled/disabled.)

That sidebar link leads to the automatically generated plugin show settings page.

spec/lib/plugin/instance_spec.rb

@@ -33,7 +33,7 @@ TEXT
     end
 
     it "defaults to using the plugin name with the discourse- prefix removed" do
-      expect(plugin_instance.humanized_name).to eq("sample-plugin")
+      expect(plugin_instance.humanized_name).to eq("Sample plugin")
     end
 
     it "uses the plugin setting category name if it exists" do
@@ -43,7 +43,7 @@ TEXT
 
     it "the plugin name the plugin site settings are still under the generic plugins: category" do
       plugin_instance.stubs(:setting_category).returns("plugins")
-      expect(plugin_instance.humanized_name).to eq("sample-plugin")
+      expect(plugin_instance.humanized_name).to eq("Sample plugin")
     end
 
     it "removes any Discourse prefix from the setting category name" do