Use sans-serif font everywhere

This change was requested for simplicity and accessibility.

Author: adamzap

djangoproject/scss/_style.scss

@@ -13,7 +13,7 @@ CSS rendered with Libsass 0.7.0
 
 //------------------- Globals
 body {
-    @include serif;
+    @include sans-serif;
     @include font-size(18);
     background: var(--sidebar-bg);
     color: var(--body-fg);
@@ -636,7 +636,6 @@ header {
     }
 
     p {
-        @include serif;
         @include font-size(18);
         color: var(--white-color);
         left: -9999px;

djangoproject/scss/_utils.scss

@@ -114,10 +114,6 @@ $logo-bg-dark: #272c27;
 }
 
 // Font Family Mixins
-@mixin serif {
-    font-family: Palatino, "Palatino Linotype", "Book Antiqua", "Hoefler Text", Georgia, "Lucida Bright", Cambria, Times, "Times New Roman", serif;
-}
-
 @mixin sans-serif {
     font-family: "Roboto", Corbel, Avenir, "Lucida Grande", "Lucida Sans", sans-serif;
 }

djangoproject/templates/styleguide.html

@@ -104,7 +104,7 @@ <h3 id="colors-secondarycolor">Secondary colors</h3>
     <h2>Typography</h2>
 
     <p>
-      The djangoproject.com website is set in both serif and sans-serif fonts. The serif font is <em>Palatino</em>, which is widely available as a system font. The sans-serif font is <em><a href="https://fonts.google.com/specimen/Roboto">Roboto</a></em>, a web font that is loaded via Google Fonts. 5 styles of this typeface are loaded, <em>Light (300)</em>, <em>Normal (400)</em>, <em>Normal Italic</em>, <em>Bold (700)</em> and <em>Bold Italic</em>. Refrain from using weights and italics that are not loaded.
+      The djangoproject.com website is set in <em><a href="https://fonts.google.com/specimen/Roboto">Roboto</a></em>, a sans-serif font. 5 styles of this typeface are provided, <em>Light (300)</em>, <em>Normal (400)</em>, <em>Normal Italic</em>, <em>Bold (700)</em> and <em>Bold Italic</em>. Refrain from using weights and italics that are not loaded.
     </p>
 
     <p>