[WIP] Allow users to add/edit similar artists

- [x] Add artist page
- [ ] <strike>Add page for user to edit artist</strike>
- [ ] Add page for user to edit/add artist similarities
This `AuthenticatedRouteMixin` makes `/artists/163/similarities` redirect to the login page when the user isn't authenticated.  However, after login the user is taken to the homepage instead of back to the previous `/artists/163/similarities` page.  I'm not sure how to fix that yet.