minor #727 Revamped the data fixtures (javiereguiluz)

This PR was squashed before being merged into the master branch (closes #727).

Discussion
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Revamped the data fixtures

After #724 and #725 I realized that our fixtures are not following the modern best practices recommended by DoctrineFixturesBundle ([explained here](https://symfony.com/doc/master/bundles/DoctrineFixturesBundle/index.html)):

* Use 1 class to load all fixtures unless you have lots of fixtures.
* Make the fixture class extend from Symfony's `Fixture` class instead of the complicated Doctrine's classes.
* Inject services in the constructor and rely on autowiring.
* etc.

Commits
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adb76fc2 Revamped the data fixtures