refactor(auth): change contributor's role logic to have it included in the users_podcasts table

• update myth-auth and codeigniter to latest develop changes
• improve permission check: remove all dynamic permissions per podcast and overwrite myth-auth services and permission filter
• remove unnecessary code because of myth-auth upgrade
• refactor some controller code for better clarity
• add remaining seeders in docs

closes #19 (closed), fixes #20 (closed)