merging tags

Merging an existing tag into another one

- [x] test coverage
- [x] finish the code
- [ ] gather feedback for my changes
- [x] submit changes to the documentation


| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | yes
| Fixed tickets | no
| Doc           | https://github.com/sulu-cmf/docs/blob/master/detail-specification/100-basic/DET-103-Tags.md