Snack bars should stack on top of each other instead of overlapping

[viviandien]

Steps to reproduce:

1. Upload PCPs file in Providers drawer.
   https://drive.google.com/open?id=1x31zlVpEi6QPC1xQl5XT1SZBzIaUiBJ4

Expected behavior:
Two snack bars appear, 1 to give an error about the data that was uploaded, and 1 to say that providers have successfully geocoded.

Actual behavior:
Both messages should be visible. Suggested UI - stack the snack bars vertically on top of each other.