feat(rss): generate rss feed from podcast entity

• refactor episode, podcast and category entities to add dynamic properties
• refactor Routes when adding feed route
• update migration files to better fit itunes' and rss' specs
• update podcast and episode forms
• add SimpleRSSElement class to Libraries
• add rss_helper
• update home controller to redirect if system has only one podcast
• update app timezone to use Coordinated Universal Time (UTC)