[FEAUTRE] seo tab counters and excerpt

__Tasks:__

- [x] add counters and the google excerpt to the seo tab
- [ ] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | none
| Fixed tickets | none
| Doc           | none

Merge before this PR:
- https://github.com/sulu-cmf/SuluAdminBundle/pull/105