Tracking: restore functionality broken by Spotify API changes

[prefixFelix]

Spotify's API changes broke a large part of psst. Several developer Web API endpoints were retired and the api-partner (pathfinder GraphQL) began rejecting psst's OAuth developer token. This issue tracks the status of restoring or replacing each affected area.

General:

• partner_auth() Login5 bearer + client-token for api-partner (fix(webapi): api-partner auth (pathfinder GraphQL) and restore home #745)
• Better error handling / display for pathfinder GraphQL

Artist page:

• Restore discography (feat(webapi): fetch artist discography via pathfinder GraphQL #746)
• Restore info & related artists (feat(webapi): fetch artist info & related artists via pathfinder GraphQL #747)
• (Add a "Load more" button for discography)
• Remove profile.externalLinks, field gone

Home page:

• Restore home page (fix(webapi): api-partner auth (pathfinder GraphQL) and restore home #745)
• Make generated ("algorithmic") playlists fetchable again (fix(webapi): fall back to pathfinder GraphQL for playlists that 404 #754)
• Fix intermittent missing field 'data' on startup

Shows / Podcasts:

• Make show/podcasts episodes load again (fix(webapi): build show episodes from the show's own episode list #755)

Songs:

• Add "Song Radio" (related to generated playlists, would also add Add support for radio's #189)
• Remove deprecated "Show Similar Tracks", endpoint gone