Warn the user before trying to draw a sample metadata barplot with more than a certain number of groups

[fedarko]

This is related to the root cause of #343. Once there are more than, say, 100 unique values in a sample metadata field, its usefulness in a stacked barplot is probably pretty low. It'd be nice to pop up a toast message or something that says "Hey this field has X unique values, are you sure you want to do this? It might take a long time." -- that way if/when the page crashes, it's clear to the user why it crashed.

[kwcantrell]

related to #381