black twitter icon in nav bar (#27)

* black twitter icon in nav bar

I think it could look more cohesive in black. I also think black is supposed to be the default so not clear why it's in blue.

* use bird instead of box for twitter

the bird icon can be styled and defaults to black

Author: story645

mpl_sphinx_theme/mpl_icon_links.html

@@ -43,7 +43,7 @@
         </li>
         <li class="nav-item">
           <a class="nav-link" href="https://twitter.com/matplotlib/" rel="noopener" target="_blank" title="twitter">
-            <span><i class="fab fa-twitter-square"></i></span>
+            <span><i class="fab fa-twitter"></i></span>
             <label class="sr-only">twitter</label>
           </a>
         </li>