Extract has_many for Channel#subscriptions
Extract has_many for Channel#playlists
Extract has_many for Playlist#playlist_items
Extract has_many for Channel#earnings and #views
Extract has_many: ContentOwner#partnered_channels
Extract has_many for Channel#videos
Extract has_many for Video#annotations
Remove duplication in `has_many` associations

Six `has_many` associations were defined in the same way, so the
code can be simplified by declaring `has_many` programmatically.

This commit does not change any functionality.
Merge branch 'extracting-has-many'