Try: Improve typographic rhythm

This PR is intended to explore adopting a change that is being explored for mobile Gutenberg in wordpress-mobile/gutenberg-mobile#550. It simply changes the lineheight from 1.8 to 1.6.

It is very important to note that this change affects only the vanilla Gutenberg style — themes can, through editor styles, still customize this to their hearts content.

Thoughts?

assets/stylesheets/_variables.scss

@@ -10,7 +10,7 @@ $editor-font: "Noto Serif", serif;
 $editor-html-font: Menlo, Consolas, monaco, monospace;
 $editor-font-size: 16px;
 $text-editor-font-size: 14px;
-$editor-line-height: 1.8;
+$editor-line-height: 1.6;
 $big-font-size: 18px;
 $mobile-text-min-font-size: 16px; // Any font size below 16px will cause Mobile Safari to "zoom in"