Csanád B

This branch is based on the WIP `chapter-15-tests` https://github.com/hjwp/Book-TDD-Web-Dev-Python/pull/264

Broken code listing id-s in Chapter 13

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The code listings' id-s in Chapter 13 refer to Chapter 11, e.g.: `(ch11l018)` When I added another, I chose to just write `(ch13l???)`.

Csanád address ch 13 review comments

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TODO: 1. "Well, for http://bit.ly/2v3SfRq[slightly counterintuitive historical reasons], Django models don't run full validation on save. // CSANAD: I'd argue against the use of shortened URLs." => https://groups.google.com/g/django-developers/c/uIhzSwWHj4c ? Check...