Merge A-Record and by-IP views to a more user-friendly way

Signed-off-by: Nicolas Iooss <nicolas.iooss_git@polytechnique.org>
urls.py: Add default handlers for 404 and 500 errors

Signed-off-by: Nicolas Iooss <nicolas.iooss_git@polytechnique.org>
Update test fixtures

Signed-off-by: Nicolas Iooss <nicolas.iooss_git@polytechnique.org>
Improve settings management

Signed-off-by: Nicolas Iooss <nicolas.iooss_git@polytechnique.org>