The contract with the frontend states that bounds should come back as an
empty array instead of null when there are no bounds present. We must
explicitly specify []string{} for this to happen.
Certain aspects of the frontend requires the presence of these three
axes, so part of the contract established is that the backend will
always provide them. Since we centralize creation of
dashboardCellResponses, this is where these axes are added to all cell
responses.
Additionally, because there was previously no coverage over the
dashboard cells endpoints, a test has been added to cover the
DashboardCells method of Service.
For the forseeable future, we will only be using the "x", "y", and "y2"
axes, even though the underlying serialization can support arbitrary
axes (for the future).
This ensures that only "x", "y", and "y2" axes are present and updates
the Swagger docs to reflect that fact