Remove Login Dropdown when Auth Views are not registered.

Fixes #1738
Merge branch 'master' of github.com:tomchristie/django-rest-framework into login-dropdown
Fix docstring
Fix imports
Moved li tags inside `optional_login`

as per https://github.com/tomchristie/django-rest-framework/pull/1820#discussion_r16987993
Prefer `format` and use named blocks
Merge pull request #1820 from carltongibson/login-dropdown

Hide login link in browsable API if the login view is not registered.