feat: Search icon in masthead is a Font Awesome icon. (mmistakes#2774)

* feat: Allow search icon in masthead to be set to a Font Awesome icon.

* fix indentations

* Users wishing to avoid FontAwesome should override _includes/masthead.html

Author: rhwood

_includes/masthead.html

@@ -21,9 +21,7 @@
         {% if site.search == true %}
         <button class="search__toggle" type="button">
           <span class="visually-hidden">{{ site.data.ui-text[site.locale].search_label | default: "Toggle search" }}</span>
-          <svg class="icon" width="16" height="16" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 15.99 16">
-            <path d="M15.5,13.12L13.19,10.8a1.69,1.69,0,0,0-1.28-.55l-0.06-.06A6.5,6.5,0,0,0,5.77,0,6.5,6.5,0,0,0,2.46,11.59a6.47,6.47,0,0,0,7.74.26l0.05,0.05a1.65,1.65,0,0,0,.5,1.24l2.38,2.38A1.68,1.68,0,0,0,15.5,13.12ZM6.4,2A4.41,4.41,0,1,1,2,6.4,4.43,4.43,0,0,1,6.4,2Z" transform="translate(-.01)"></path>
-          </svg>
+          <i class="fas fa-search"></i>
         </button>
         {% endif %}
         <button class="greedy-nav__toggle hidden" type="button">