Migrate Jade to Pug template engine

Jade templates engine has been renamed to Pug.

We are using Pug already on the Blender Cloud repository, following is Flamenco and Attract

src/templates/errors/403_embed.pug

@@ -1,4 +1,4 @@
-#error-container(class="error-403")
+#error-container.error-403
 	#error-box
 		.error-top-container
 			.error-title Forbidden.

src/templates/errors/404_embed.pug

@@ -1,4 +1,4 @@
-#error-container(class="error-404")
+#error-container.error-404
 	#error-box
 		.error-top-container
 			.error-title Not found.

src/templates/errors/412.pug

@@ -1,6 +1,6 @@
 | {% extends "errors/layout.html" %}
 | {% block body %}
-#error_container.412.standalone
+#error_container.error-412.standalone
 	#error_box
 		.error-title 412. Precondition Failed.
 

src/templates/errors/412_embed.pug

@@ -1,4 +1,4 @@
-#error_container.412
+#error_container.error-412
 	#error_box
 		.error-title 412. Precondition Failed.
 

src/templates/errors/500.pug

@@ -21,7 +21,7 @@ html(lang="en")
 
 	body.error
 
-		#error_container.500.standalone
+		#error_container.error-500.standalone
 			#error_box
 				.error-top-container
 					.error-title Something went wrong.

src/templates/errors/500_embed.pug

@@ -1,4 +1,4 @@
-#error_container.500
+#error_container.error-500
 	#error_box
 		.error-top-container
 			.error-title Something went wrong.