Support changing colors from within a legend

This would involve doing what Emperor does: rather than drawing just a colored square for each category we'd add a Spectrum.js colorpicker.

https://github.com/biocore/empress/blob/5d19f0dfd244b95d1ff60f3dc9a197db2bbe66c0/empress/support_files/js/legend.js#L145-L150

Actually adding the colorpickers likely wouldn't be too bad (although if a field has like a thousand groups it might slow down the page, see e.g. #344, so we may want to only create a colorpicker when a field is clicked on by the user) -- I think the hard part will likely be refactoring the code to use the selected colors efficiently? Like, preferably we'd store the `Colorer` created for a given field and just modify that, rather than calling `colorByFeatureMetadata` / `colorBySampleCat` again. Or maybe that wouldn't be too slow, I dunno.

This would impact:

- sample metadata coloring
- feature metadata coloring
- barplots (categorical feature metadata / stacked sample metadata)
- animations (I think colors are assigned based on the trajectory? We could probably draw a legend for the trajectory in the side panel, similarly to how barplot legends are drawn, so that users can manipulate it there.)