Show suggestion on the 404 page
use slug
add tests for helpful 404 page
separate getting suggestion logic from error handler
handle fallbacks in URL dispatcher
remove unused import
Merge branch 'master' into feature/rich404

Conflicts:
	readthedocs/templates/404.html
load core_tag
fix doc_url tag to return a valid URL
add tests for doc_url tag
doc_url tag supports singlehtml
Merge remote-tracking branch 'maskit/feature/rich404'

Conflicts:
	readthedocs/templates/404.html