Google Analytics for WO & WL

These changes enable creation of google analytics
reports around `WorldwideOrganisation` and
`WorldLocation`.

#### Changes

- add `analytics_identifier` to these models
- ensure `analytics_identifier` is populated after create
- pass relevant slimmer headers to set-up GA `customVars`:
  - `Organisations` (for Worldwide organisation)
  - `WorldLocations`
- we've made a change on slimmer to add WorldLocations
`customVar`, and used that version here.

#### WorldLocations customVar now appears on

- `WorldLocation` landing page
- `WorldwideOrganisation` landing page
- These editions if they belong to a WorldLocation
  - `CaseStudy`
  - `FatalityNotice`
  - `WorldwidePriority`
  - `Policy`
  - `WorldLocationNewsArticle`
  - `Speech`
  - `NewsArticle`
  - `Publication`

#### Organisations customVar now includes Worldwide Organisation on

- `WorldwideOrganisation` landing page
- These editions if they belong to a Worldwide Organisation
  - `CaseStudy`
  - `WorldwidePriority`
  - `WorldLocationNewsArticle`