Prepare Mastodon for zeitwerk autoloader (#15917)

* Prepare Mastodon for zeitwerk autoloader (Rails 6)

Add inflections and rename/move a few classes.

In particular, app/lib/exceptions.rb and app/lib/sanitize_config.rb
were manually loaded while still in autoload paths.

* Add inflection for Url → URL

lib/sanitize_ext/sanitize_config.rb

@@ -0,0 +1,119 @@
+# frozen_string_literal: true
+
+class Sanitize
+  module Config
+    HTTP_PROTOCOLS = %w(
+      http
+      https
+    ).freeze
+
+    LINK_PROTOCOLS = %w(
+      http
+      https
+      dat
+      dweb
+      ipfs
+      ipns
+      ssb
+      gopher
+      xmpp
+      magnet
+      gemini
+    ).freeze
+
+    CLASS_WHITELIST_TRANSFORMER = lambda do |env|
+      node = env[:node]
+      class_list = node['class']&.split(/[\t\n\f\r ]/)
+
+      return unless class_list
+
+      class_list.keep_if do |e|
+        next true if /^(h|p|u|dt|e)-/.match?(e) # microformats classes
+        next true if /^(mention|hashtag)$/.match?(e) # semantic classes
+        next true if /^(ellipsis|invisible)$/.match?(e) # link formatting classes
+      end
+
+      node['class'] = class_list.join(' ')
+    end
+
+    UNSUPPORTED_HREF_TRANSFORMER = lambda do |env|
+      return unless env[:node_name] == 'a'
+
+      current_node = env[:node]
+
+      scheme = begin
+        if current_node['href'] =~ Sanitize::REGEX_PROTOCOL
+          Regexp.last_match(1).downcase
+        else
+          :relative
+        end
+      end
+
+      current_node.replace(current_node.text) unless LINK_PROTOCOLS.include?(scheme)
+    end
+
+    UNSUPPORTED_ELEMENTS_TRANSFORMER = lambda do |env|
+      return unless %w(h1 h2 h3 h4 h5 h6 blockquote pre ul ol li).include?(env[:node_name])
+
+      current_node = env[:node]
+
+      case env[:node_name]
+      when 'li'
+        current_node.traverse do |node|
+          next unless %w(p ul ol li).include?(node.name)
+
+          node.add_next_sibling('<br>') if node.next_sibling
+          node.replace(node.children) unless node.text?
+        end
+      else
+        current_node.name = 'p'
+      end
+    end
+
+    MASTODON_STRICT ||= freeze_config(
+      elements: %w(p br span a),
+
+      attributes: {
+        'a'    => %w(href rel class),
+        'span' => %w(class),
+      },
+
+      add_attributes: {
+        'a' => {
+          'rel' => 'nofollow noopener noreferrer',
+          'target' => '_blank',
+        },
+      },
+
+      protocols: {},
+
+      transformers: [
+        CLASS_WHITELIST_TRANSFORMER,
+        UNSUPPORTED_ELEMENTS_TRANSFORMER,
+        UNSUPPORTED_HREF_TRANSFORMER,
+      ]
+    )
+
+    MASTODON_OEMBED ||= freeze_config merge(
+      RELAXED,
+      elements: RELAXED[:elements] + %w(audio embed iframe source video),
+
+      attributes: merge(
+        RELAXED[:attributes],
+        'audio'  => %w(controls),
+        'embed'  => %w(height src type width),
+        'iframe' => %w(allowfullscreen frameborder height scrolling src width),
+        'source' => %w(src type),
+        'video'  => %w(controls height loop width),
+        'div'    => [:data]
+      ),
+
+      protocols: merge(
+        RELAXED[:protocols],
+        'embed'  => { 'src' => HTTP_PROTOCOLS },
+        'iframe' => { 'src' => HTTP_PROTOCOLS },
+        'source' => { 'src' => HTTP_PROTOCOLS }
+      )
+    )
+  end
+end