Heatmap Compare-Results: Get assessment data

[rolfheij-sil]

Besides the 'basic' heatmap results (where two projects are being compared) AQuA also has a 'Compare Results' heatmap, that shows the texts compared to a set of baseline projects.

This issue is about getting the data for this feature.
Issue #29 is about creating the UI for it.

The data that is needed for this heatmap can be fetched from this endpoint

Note that this endpoint is basically the same as the 'regular' results endpoint, but has an optional parameter baseline_ids. If you want you can refactor the code so that it always uses the compareresults endpoint.