Redundant assets in template locals

[smashwilson]

The context used to render Nunjucks templates includes global assets in two places:

• deconst.content.assets which includes the assets injected during envelope rendering by the content service.
• deconst.assets includes the assets fetched from the content service explicitly at the /assets endpoint.

I tend to prefer the latter as it's a little shorter to reference within templates, a little cleaner in terms of architecture, and most of our "real" templates are already using it. Getting rid of the former should be a matter of doing some final grepping in nexus-control, deleting some code in the content service, and updating our documentation.