[WIP] Use Doctrine fixtures to load test data

Related to https://github.com/protalk/protalk/issues/162, I've started working on a way to import entities to the database using Doctrine fixtures both for tests and as a replacement to importing data using a .sql file because it's easier to maintain with database schema changes and easier to get a working database up and running with real data.

I've added at least one fixture for all the `Protalk\MediaBundle` entities. There's several speaker entities and a couple of videos in there right now. If this is acceptable then I'll add many more entities of each type and then do the same for the `Protalk\PageBundle` too.

The docs to understand how loading dependencies through fixtures and referencing them across classes is on the [doctrine/data-fixtures](https://github.com/doctrine/data-fixtures) bundle.