Use `search_text_nodes` helper in our custom filters

Closes #1477

lib/gitlab/markdown/emoji_filter.rb

@@ -15,7 +15,7 @@ module Gitlab
       IGNORED_ANCESTOR_TAGS = %w(pre code tt).to_set
 
       def call
-        doc.search('text()').each do |node|
+        search_text_nodes(doc).each do |node|
           content = node.to_html
           next unless content.include?(':')
           next if has_ancestor?(node, IGNORED_ANCESTOR_TAGS)

lib/gitlab/markdown/reference_filter.rb

@@ -49,7 +49,7 @@ module Gitlab
       def replace_text_nodes_matching(pattern)
         return doc if project.nil?
 
-        doc.search('text()').each do |node|
+        search_text_nodes(doc).each do |node|
           content = node.to_html
 
           next unless content.match(pattern)