[FEATURE] Register Content Types by DI tags

- Refactored content type manager to accept alias => service ID
  registrations.
- All content types must now use the `sulu.content.type` tag.

I choose to overwrite the existing ContentTypeManager and not provide any BC. We would have to update all the existing content types to use this new method.

- [x] PR ContentBundle: https://github.com/sulu-cmf/SuluContentBundle/pull/148
- [x] PR MediaBundle: https://github.com/sulu-cmf/SuluMediaBundle/pull/32
- [x] PR TagBundle: https://github.com/sulu-cmf/SuluTagBundle/pull/37
- [x] PR CategoryBundle: https://github.com/sulu-cmf/SuluCategoryBundle/pull/7

- [ ] Gather feedback for my changes ..
- [ ] Tests pass? No -- not until we merge all of the above PRs

Yeah, I noticed that. But I like them - it means your putting your name to a piece of code, so it is representative of your work - and it is also a reason to ensure that the code doesn't get worse and so make you look bad :)