Automated testing framework

Hello,

It would be great to add a testing framework to the project. Beyond Django **unittests** I suggest a behaviour-driven (BDD) component : [Behave](https://pypi.org/project/behave/). There's an available [Django integration](https://behave-django.readthedocs.io/en/stable/), and has as main advantage that tests can be easily written, understood, and enhances code-reuse (which is a way to avoid _test code rot_).

What a simple test might look like :
```
Feature : Minting ark identifiers
Scenario: Simple minting request
Given we have a running arklet service
When we mint an id for url "http://example.com/this_test_is_ok"
Then we obtain an ARK id
And it forwards to "http://example.com/this_test_is_ok"
```
(see [behave](https://pypi.org/project/behave/) documentation for more information)

If the suggestion is validated, I can work on a PR that implements testing (be it behave or unittests).