Revamped the data fixtures

[javiereguiluz]

After #724 and #725 I realized that our fixtures are not following the modern best practices recommended by DoctrineFixturesBundle (explained here):

• 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.

[yceruto]

I like this revamp, but we might need to keep the FixturesTrait into a separate file because it's used by tests/Controller/Admin/BlogControllerTest.php avoiding code duplication.

[javiereguiluz]

@yceruto you are right. However, when I was going to do that change, I realized that we barely use the fixtures trait in tests. So, what do you think if we just replace the entire fixtures trait just by the the generateRandomString() method added in 7c3e865?

[yceruto]

Perfect to me!