Issue 703 fix error templates

This PR converts the 404 and 500 error pages from static HTML to Django templates, using the same base.html template that all other RTD pages use.

This fixes the broken links in the header (by prepending PRODUCTION_DOMAIN to the links), and also uses MEDIA_URL to serve the js and css files, rather than using hard-coded links to http://media.readthedocs.org.

As far as I can tell, error pages are always served by Django, and never by Nginx directly, so converting them to Django templates is a good solution. 

If that's not the case, or if there is another reason why the error pages need to be static HTML (rather than Django templates), then merging this PR is a bad idea!