Enable headless policies

Enables **`policy(:dashboard) # => DashboardPolicy`**. 

Policies without a matching model can come in handy when a controller
isn't modeled alongside a resource, e.g. a `DashboardsController`.

The policy lookup by symbol also helps with strong parameters,
since I prefer `policy(:post)` or `policy(@post || :post)` over `policy(@post || Post)`.