views: add embed_wiki helper.

Also remove options to configure names of wiki notice pages. These names
generally don't need to be changed and we already hardcode links to wiki
pages in other places anyway.

app/controllers/uploads_controller.rb

@@ -5,7 +5,6 @@ class UploadsController < ApplicationController
 
   def new
     @source = Sources::Strategies.find(params[:url], params[:ref]) if params[:url].present?
-    @upload_notice_wiki = WikiPage.titled(Danbooru.config.upload_notice_wiki_page).first
     @upload, @remote_size = UploadService::ControllerHelper.prepare(
       url: params[:url], ref: params[:ref]
     )