FEATURE: Inline (Mini) Oneboxing

see:
https://meta.discourse.org/t/mini-inline-onebox-support-rfc/66400?source_topic_id=66066

lib/inline_oneboxer.rb

@@ -0,0 +1,47 @@
+class InlineOneboxer
+
+  def initialize(urls)
+    @urls = urls
+  end
+
+  def process
+    @urls.map {|url| InlineOneboxer.lookup(url) }.compact
+  end
+
+  def self.clear_cache!
+  end
+
+  def self.cache_lookup(url)
+    Rails.cache.read(cache_key(url))
+  end
+
+  def self.lookup(url)
+    cached = cache_lookup(url)
+    return cached if cached.present?
+
+    if route = Discourse.route_for(url)
+      if route[:controller] == "topics" &&
+        route[:action] == "show" &&
+
+        topic = Topic.where(id: route[:topic_id].to_i).first
+
+        # Only public topics
+        if Guardian.new.can_see?(topic)
+          onebox = { url: url, title: topic.title }
+          Rails.cache.write(cache_key(url), onebox, expires_in: 1.day)
+          return onebox
+        end
+      end
+    end
+
+    nil
+  end
+
+  private
+
+    def self.cache_key(url)
+      "inline_onebox:#{url}"
+    end
+
+end
+