FEATURE: Reimplement SiteSetting.max_oneboxes_per_post. (#6668)

Previously, the site setting was only effective on the client side of
things. Once the site setting was been reached, all oneboxes are not
rendered. This commit changes it such that the site setting is respected
both on the client and server side. The first N oneboxes are rendered and
once the limit has been reached, subsequent oneboxes will not be
rendered.

lib/oneboxer.rb

@@ -5,6 +5,8 @@ require_dependency 'final_destination'
 Dir["#{Rails.root}/lib/onebox/engine/*_onebox.rb"].sort.each { |f| require f }
 
 module Oneboxer
+  ONEBOX_CSS_CLASS = "onebox"
+
   # keep reloaders happy
   unless defined? Oneboxer::Result
     Result = Struct.new(:doc, :changed) do
@@ -67,11 +69,11 @@ module Oneboxer
   end
 
   # Parse URLs out of HTML, returning the document when finished.
-  def self.each_onebox_link(string_or_doc)
+  def self.each_onebox_link(string_or_doc, extra_paths: [])
     doc = string_or_doc
     doc = Nokogiri::HTML::fragment(doc) if doc.is_a?(String)
 
-    onebox_links = doc.search("a.onebox")
+    onebox_links = doc.css("a.#{ONEBOX_CSS_CLASS}", *extra_paths)
     if onebox_links.present?
       onebox_links.each do |link|
         yield(link['href'], link) if link['href'].present?
@@ -83,13 +85,14 @@ module Oneboxer
 
   HTML5_BLOCK_ELEMENTS ||= %w{address article aside blockquote canvas center dd div dl dt fieldset figcaption figure footer form h1 h2 h3 h4 h5 h6 header hgroup hr li main nav noscript ol output p pre section table tfoot ul video}
 
-  def self.apply(string_or_doc, args = nil)
+  def self.apply(string_or_doc, extra_paths: nil)
     doc = string_or_doc
     doc = Nokogiri::HTML::fragment(doc) if doc.is_a?(String)
     changed = false
 
-    each_onebox_link(doc) do |url, element|
+    each_onebox_link(doc, extra_paths: extra_paths) do |url, element|
       onebox, _ = yield(url, element)
+
       if onebox
         parsed_onebox = Nokogiri::HTML::fragment(onebox)
         next unless parsed_onebox.children.count > 0