Using a better system font stack

Inspired by http://markdotto.com/2018/02/07/github-system-fonts/

packages/@uppy/core/src/_variables.scss

@@ -1,6 +1,5 @@
 // Fonts
-$font-family-base: -apple-system, BlinkMacSystemFont, Segoe UI, Roboto,
-                   Oxygen, Ubuntu, Cantarell, Fira Sans, Droid Sans, Helvetica Neue, sans-serif !default;
+$font-family-base: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol' !default;
 
 // Colors