Accept `where` in ViewerPercentages collection.

This prevents N+1 queries when retrieving viewer percentages for
multiple partnered channels at once.
`partnered_channels.includes :viewer_percentages`

`content_owner.partnered_channels` now accepts a chainable `includes`
method that, when called with `:viewer_percentages`, eagerly loads all
the viewer percentages of the partnered channels, so they can be
accessed without N+1 queries.