CategoryModel.php

- add podcast sidebar navigation
- add podcast dashboard with latest episodes
- add pagination to podcast episodes
- add components helper to reuse ui components (button, data_table, etc.)
- enhance podcast and episode forms by splitting them into form sections
- add hint tooltips to podcast and episode forms
- transform radio inputs as buttons for better ux
- replace explicit field by parental_advisory
- replace author field by publisher
- add podcasts_categories table to set multiple categories
- use choices.js to enhance multiselect fields
- update Language files
- update js dependencies to latest versions

closes #31, #9