[#77608474] Implement dashboard `GET`

## Nicer implementation pattern suggestions welcome

- The tests don'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.
- 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, which feels like I'm leaving us with a curious beaƒt

![](http://ebba.english.ucsb.edu/images/cache/hunt_1_18306_2448x2448.jpg)
* `GET /dashboard` should return a representation of the list of dashboards
* `GET /dashboard/{id}` should return a specific dashboard or 404
* `PUT /dashboard/{id}` should update a specific dashboard
* `POST /dashboard` should create a new dashboard

`urls.py` could be altered to be more in line with that, or maybe take a look at [something else](https://github.com/dnerdy/django-reroute).