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

app/Models/EpisodeModel.php

@@ -18,17 +18,18 @@ class EpisodeModel extends Model
         'podcast_id',
         'title',
         'slug',
-        'enclosure_url',
+        'enclosure_uri',
         'enclosure_length',
         'enclosure_type',
-        'guid',
         'pub_date',
         'description',
         'duration',
-        'image',
+        'image_uri',
         'explicit',
         'number',
         'season_number',
+        'author_name',
+        'author_email',
         'type',
         'block',
     ];