Visualizations: make custom legends with check boxes to de-select overlay values

[danicahelb]

In phase 1, users are able to click on each of the overlay values to deselect and remove them from the plot. The text for the de-selected value turns grey.

In phase 2, we want to improve functionality. We can put check boxes in front of each overlay value so that the user intuitively knows that the values can be selected or de-selected.

We have requested that--when an overlay value has no data because of subsetting--that value should be shown in grey text. https://github.com/VEuPathDB/web-eda/issues/339

To distinguish values with no data from values that have data but have been de-selected, perhaps the de-selected values should remain in black text but the check box for them won't be checked.

[danicahelb]

this duplicates https://github.com/VEuPathDB/web-eda/issues/579, so i am closing it now