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/_modal.pug

@@ -0,0 +1,13 @@
+// Modal
+#modal-default.modal.fade(tabindex='-1', role='dialog', aria-labelledby='modal-default-label')
+	.modal-dialog(role='document')
+		.modal-content
+			.modal-header
+				button.close(type='button', data-dismiss='modal', aria-label='Close')
+					span(aria-hidden='true') ×
+				h4#modal-default-label.modal-title Modal title
+			.modal-body
+				| ...
+			.modal-footer
+				button.btn.btn-default(type='button', data-dismiss='modal') Close
+				button.btn.btn-primary(type='button') Save changes