Consistently show the `context` param as an endpoint argument for GET requests

In `WP_REST_Controller->get_collection_params()` we define the top-level collection params:
- `page`
- `per_page`
- `search`

We should add to that the `context` param as well, so we have it consistently in the collection params.  

Additionally, I don't think we define the params for our single item endpoints.  We should communicate the context param there as well.
