ref(css): Shrink h4 to differentiate it from h3 (#1675)

As pointed out in #1667 (comment), actual headings have many advantages over text made to look like a heading by bolding it and putting it on its own line.

In markdown, it's the difference between `#### Dogs Rule` and `**Dogs Rule**`. In HTML it means anchor links, consistent styling, and better accessibility (vs. lack of all of those things).

So, a clear win for headings, right? It would be, except that with our current CSS, it's next to impossible to tell an `h4` (`#### Dogs Rule`) from an `h3` (`### Dogs Rule`), because the difference is only a 4% reduction in size. Since using `h4`s breaks the document's visual hierarchy, folks in many cases choose to use the fake heading in spite of its drawbacks, because it _is_ visually distinct from an `h3`.

This shrinks the size of `h4`s so that you get the look of the fake heading, with all of the advantages of the real one.

Author: lobsterkatie

src/_assets/css/_includes/bootstrap-overrides.scss

@@ -5,7 +5,7 @@ $small-font-size: 85%;
 $h1-font-size: 2em;
 $h2-font-size: 1.5em;
 $h3-font-size: 1.125em;
-$h4-font-size: 1.075em;
+$h4-font-size: 1em;
 $h5-font-size: 1em;
 $h6-font-size: 1em;