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

@@ -0,0 +1,35 @@
+#error-container.error-403
+	#error-box
+		.error-top-container
+			.error-title Forbidden.
+			.error-lead
+				p You don't have access to this content.
+
+				| {% if current_user.is_authenticated %}
+				| {% if current_user.has_role('demo') %}
+				| {% set subscription = 'demo' %}
+				| {% elif current_user.has_cap('subscriber') %}
+				| {% set subscription = 'subscriber' %}
+				| {% else %}
+				| {% set subscription = 'none' %}
+				| {% endif %}
+
+				| {% if subscription == 'none' %}
+				hr
+				p
+					| Support Blender and get awesome stuff. 
+					a(href="https://store.blender.org/product/membership/") Subscribe Now
+				| {% endif %}
+
+				| {% else %}
+				p
+					a(href="{{ url_for('users.login') }}") Login
+				| {% endif %}
+
+		.error-lead.extra
+			| {% if current_user.is_authenticated %}
+			p.
+				If you have just subscribed, please <a href="{{ url_for('users.logout') }}">log out</a> and in again.
+			| {% endif %}
+			p.
+				Need help? Get in touch via <a href="mailto:cloudsupport@blender.org">e-mail</a>.