Endpoint to get the dashboards using a dataset

When we are building notification emails we need to know what dashboards
the dataset that is out of date is used in, such that we can provide a
slightly more human friendly message tot the end user.
Define a thing as a constant

It’s used as a constant, so define it as such.
Merge pull request #267 from alphagov/feature/dashboards-for-dataset

Endpoint to get the dashboards using a dataset
Merge pull request #269 from alphagov/idiomatic-python

Define a thing as a constant