FEATURE: Show more context in Discourse topic oneboxes

Currently when generating a onebox for Discourse topics, some important
context is missing such as categories and tags.

This patch addresses this issue by introducing a new onebox engine
dedicated to display this information when available. Indeed to get this
new information, categories and tags are exposed in the topic metadata
as opengraph tags.

lib/cooked_processor_mixin.rb

@@ -70,7 +70,7 @@ module CookedProcessorMixin
           found = false
           parent = img
           while parent = parent.parent
-            if parent["class"] && parent["class"].include?("allowlistedgeneric")
+            if parent["class"] && parent["class"].match?(/\b(allowlistedgeneric|discoursetopic)\b/)
               found = true
               break
             end