[#77608474] Add `DashboardViewsGetTestCase` to `test_dashboard.py`

- Doesn't make assertions about UUIDs, as they constantly seem to change. **I THINK** this is by design, but would like to check before having this merged.
[#77608474] Implement dashboard `GET`

**Nicer implementation pattern suggestions welcome**

- We're still using the same url here, so the `dashboard` method routes to a `get_dashboard` method, which feels properly dumb but...
  - I didn't want to wrestle with trying to route based on the request inside `urls.py`
  - Annoyingly this means that `@require_http_methods` on the `dashboard` method now takes `(['POST', 'PUT', 'GET'])` which feels extra dumb, because we're putting routing logic in a bloody view.
Merge pull request #232 from alphagov/get_dashboard

[#77608474] Implement dashboard `GET`