Behat taxonomy scenarios

I'm sending this PR in order to get some feedback.

There is a lot of things to fix. Feature contexts are in mess right now, we need a better organization, probably customer and admin context.

We need to be able to get rid of this xpath selectors and select which taxonomy to eidt/delete by it's name. I guess the right way would be to create steps which will put me on right route depending on taonomy/taxon chosen in scenario. For example, if I say `And I follow edit taxon "Bookmania"` it should put me on `sylius_sandbox_backend_taxon_update` route.

What do you think?