Introducing a tag repository, with a few helpful methods

[weaverryan]

Hey Fabien!

This repository should be generic enough, and hopefully quite helpful. It's probably not perfect, but a good start. A few comments:

• I'm not exactly sure why we have the "name" field in the extension, but then a "slug" field in the bundle. I understand the idea of normalizing all the tags, but it seems like we should do that in the extension (not just in the bundle). To allow the bundle to work later, I've added a tagQueryField property to the TagRepository so that we eventually create a TagRepository in the bundle, which subclasses this one and changes that property to slug.
• As mentioned above, once we're happy with this, some changes should be made to the bundle (and its documentation) so that this works easily there as well.

Thanks!

[FabienPennequin]

Thanks Ryan!

[weaverryan]

@FabienPennequin What about the name and slug field? What's the purpose of having a name field in the extension, but then a slug field in the bundle?

Thanks!