DEV: Make discourse_narrative_bot use Rails autoload (#26044)

Why this change?

Instead of manually loading files, we should just structure the plugin
so that it relies on Rails autoload strategy and avoid all the manual
`require_relative`s.

What does this change do?

1. Structure the plugin to use Rails autoloading convention
2. Remove onceff jobs that were added 5-6 years ago. There is no need to
   carry these jobs anymore after such a long time.
3. Move setting of `SiteSetting.discourse_narrative_bot_enabled` to
   `false` in the test environment from core into the plugin.

plugins/discourse-narrative-bot/app/controllers/discourse_narrative_bot/certificates_controller.rb

@@ -0,0 +1,44 @@
+# frozen_string_literal: true
+
+module DiscourseNarrativeBot
+  class CertificatesController < ::ApplicationController
+    requires_plugin DiscourseNarrativeBot::PLUGIN_NAME
+    layout false
+    skip_before_action :check_xhr
+    requires_login
+
+    def generate
+      immutable_for(24.hours)
+
+      %i[date user_id].each do |key|
+        unless params[key]&.present?
+          raise Discourse::InvalidParameters.new("#{key} must be present")
+        end
+      end
+
+      if params[:user_id].to_i != current_user.id
+        rate_limiter = RateLimiter.new(current_user, "svg_certificate", 3, 1.minute)
+      else
+        rate_limiter = RateLimiter.new(current_user, "svg_certificate_self", 30, 10.minutes)
+      end
+      rate_limiter.performed! unless current_user.staff?
+
+      user = User.find_by(id: params[:user_id])
+      raise Discourse::NotFound if user.blank?
+
+      hijack do
+        generator = CertificateGenerator.new(user, params[:date], avatar_url(user))
+
+        svg = params[:type] == "advanced" ? generator.advanced_user_track : generator.new_user_track
+
+        respond_to { |format| format.svg { render inline: svg } }
+      end
+    end
+
+    private
+
+    def avatar_url(user)
+      UrlHelper.absolute(Discourse.base_path + user.avatar_template.gsub("{size}", "250"))
+    end
+  end
+end