* Use 'site_url' instead of hardcoding DRF homepage
* Use 'url' template filter instead of 'base_url'
This fixes static file loading for the 404 page.
* Only insert funding <hr> if toc is present
* Link quickstart to valid API guide page
* Fix 404 search modal link
* Use 'base_url' instead of 'site_url' on 404 page
This allows us to delete lots of code and avoid hard-coding the
navigation. This does mean that {{ current_page }} isn't always
available and needs to be conditionally used.
