statusbar: Use horizontal `ch` unit instead of vertical `em`

src/scss/_statusbar.scss

@@ -82,7 +82,7 @@
 }
 
 .uppy-StatusBar-contentPadding {
-  margin-right: 0.3em;
+  margin-right: 0.5ch; // ½ the size of a 0, roughly the size of a space usually
 }
 
 .uppy-StatusBar-status {