Minor bug fixes

Here are a bunch of fixes I needed when using this app for a project.

Probably the biggest change is removing `db_table = 'django_redirect'` on the model. I needed to use both this app and django's redirects app at the same time so that I could migrate off django's redirects app which also defines `db_table = 'django_redirect'`. It it would also work to define db_table to something other than what the django's redirects app uses.

Thanks for making this app!