Provide UI elements for clustered heatmap generation...Issue:
https://github.com/genenetwork/gn-gemtext-threads/blob/main/topics/gn1-migration-to-gn2/clustering.gmi
* wqflask/wqflask/collect.py: provide hard-coded URL for testing
* wqflask/wqflask/templates/collections/view.html: provide button
Provide a button to trigger the heatmap generation. As a test, we
also provide a hard-coded URL for the API endpoint to get the
heatmap data.