Add slug attributes to Movie, Show, Season, and Episode

[JonnyWong16]

Description

Adds slug attributes, the watch.plex.tv clean URL identifier, to:

• Movie.slug
• Show.slug
• Season.parentSlug
• Episode.grandparentSlug

Requires: Plex Media Server 1.40.0.

Type of change

Please delete options that are not relevant.

• [x] New feature (non-breaking change which adds functionality)
• [x] This change requires a documentation update

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [x] I have added or updated the docstring for new or existing methods
• [x] I have added tests when applicable