[WebsiteBundle] #585 Google Analytics UID in webspace config

Define analytics key in url in webspace

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes
- [x] submit changes to the documentation
- [x] add feature #585 


__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | 
Do we want to be adding vendor specific things in a core component?

Could we not solve this with some sort of filter? /cc @danrot @chirimoya @wachterjohannes