[security chapter] update code for Django 2.x

[ftnext]

Changes

• use path function instead of url
• use views.LoginView instead of views.login (for Django 2.1.x)

References

• source code: https://github.com/ftnext/my-first-blog/tree/e379c55423b1fe089cdfbf104013241d98caab38
• my blog(Japanese): http://nikkie-ftnext.hatenablog.com/entry/2018/11/26/232818

Background

The Japanese Django Girls community is translating the extension tutorial. We noticed that the code for Django 1.x remained, so I propose an update for Django 2.x.

[das-g]

en/homework_create_more_models would need updating, too. But that can be done in a separate PR, so should not block this one.

[ftnext]

@das-g Thank you for review!

I prepared an update of en/homework_create_more_models in another PR https://github.com/DjangoGirls/tutorial-extensions/pull/120 . I hope you will be able to review it.