Set heatmap width based on headers
Set heatmap width based on headers

When showing empty columns, the length of the longest row is not
actually the number of columns, so we should set the width based on the
headers shown.

https://seedscientific.atlassian.net/browse/POLIO-921

unicef/rhizome,unicef/rhizome,SeedScientific/polio,unicef/polio,unicef/polio,SeedScientific/polio,unicef/rhizome,unicef/rhizome,SeedScientific/polio,SeedScientific/polio,unicef/polio,SeedScientific/polio,unicef/polio