Add RSS feeds for end-users (#7259)

* Add RSS feed for accounts

* Add RSS feeds for hashtags

* Fix code style issues

* Fix code style issues

app/lib/rss_builder.rb

@@ -0,0 +1,130 @@
+# frozen_string_literal: true
+
+class RSSBuilder
+  class ItemBuilder
+    def initialize
+      @item = Ox::Element.new('item')
+    end
+
+    def title(str)
+      @item << (Ox::Element.new('title') << str)
+
+      self
+    end
+
+    def link(str)
+      @item << Ox::Element.new('guid').tap do |guid|
+        guid['isPermalink'] = 'true'
+        guid << str
+      end
+
+      @item << (Ox::Element.new('link') << str)
+
+      self
+    end
+
+    def pub_date(date)
+      @item << (Ox::Element.new('pubDate') << date.to_formatted_s(:rfc822))
+
+      self
+    end
+
+    def description(str)
+      @item << (Ox::Element.new('description') << str)
+
+      self
+    end
+
+    def enclosure(url, type, size)
+      @item << Ox::Element.new('enclosure').tap do |enclosure|
+        enclosure['url']    = url
+        enclosure['length'] = size
+        enclosure['type']   = type
+      end
+
+      self
+    end
+
+    def to_element
+      @item
+    end
+  end
+
+  def initialize
+    @document = Ox::Document.new(version: '1.0')
+    @channel  = Ox::Element.new('channel')
+
+    @document << (rss << @channel)
+  end
+
+  def title(str)
+    @channel << (Ox::Element.new('title') << str)
+
+    self
+  end
+
+  def link(str)
+    @channel << (Ox::Element.new('link') << str)
+
+    self
+  end
+
+  def image(str)
+    @channel << Ox::Element.new('image').tap do |image|
+      image << (Ox::Element.new('url') << str)
+      image << (Ox::Element.new('title') << '')
+      image << (Ox::Element.new('link') << '')
+    end
+
+    @channel << (Ox::Element.new('webfeeds:icon') << str)
+
+    self
+  end
+
+  def cover(str)
+    @channel << Ox::Element.new('webfeeds:cover').tap do |cover|
+      cover['image'] = str
+    end
+
+    self
+  end
+
+  def logo(str)
+    @channel << (Ox::Element.new('webfeeds:logo') << str)
+
+    self
+  end
+
+  def accent_color(str)
+    @channel << (Ox::Element.new('webfeeds:accentColor') << str)
+
+    self
+  end
+
+  def description(str)
+    @channel << (Ox::Element.new('description') << str)
+
+    self
+  end
+
+  def item
+    @channel << ItemBuilder.new.tap do |item|
+      yield item
+    end.to_element
+
+    self
+  end
+
+  def to_xml
+    ('<?xml version="1.0" encoding="UTF-8"?>' + Ox.dump(@document, effort: :tolerant)).force_encoding('UTF-8')
+  end
+
+  private
+
+  def rss
+    Ox::Element.new('rss').tap do |rss|
+      rss['version']        = '2.0'
+      rss['xmlns:webfeeds'] = 'http://webfeeds.org/rss/1.0'
+    end
+  end
+end