Added mask icon for Safari

This is used for pinned tabs and the touchbar on the new MacBook Pro

public/images/webkit-mask-icon.svg

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg viewBox="0 0 16 16" version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
+    <path d="M8.88291829,10.9307123 C8.14408278,11.2962793 7.30722963,11.5024614 6.42066622,11.5024614 C3.42691479,11.5024614 1,9.15140547 1,6.2512307 C1,3.35105592 3.42691479,1 6.42066622,1 C9.41441766,1 11.8413324,3.35105592 11.8413324,6.2512307 C11.8413324,7.33864188 11.5001444,8.34885514 10.9158167,9.18688691 L14.7160748,12.8683592 C14.7160748,12.8683592 15.3549065,13.4872227 14.7160748,14.1060861 L14.0772432,14.7249496 C13.4384116,15.343813 12.7995799,14.7249496 12.7995799,14.7249496 L8.88291829,10.9307123 Z M2.80688874,6.2512307 C2.80688874,8.18468059 4.42483189,9.75205116 6.42066622,9.75205116 C8.41650056,9.75205116 10.0344437,8.18468059 10.0344437,6.2512307 C10.0344437,4.3177808 8.41650056,2.75041023 6.42066622,2.75041023 C4.42483189,2.75041023 2.80688874,4.3177808 2.80688874,6.2512307 Z" />
+</svg>

views/index.erb

@@ -28,6 +28,7 @@
   <link rel="apple-touch-icon" sizes="144x144" href="<%= App.cdn_origin %>/images/apple-icon-144.png">
   <link rel="apple-touch-icon" sizes="152x152" href="<%= App.cdn_origin %>/images/apple-icon-152.png">
   <link rel="apple-touch-icon" sizes="160x160" href="<%= App.cdn_origin %>/images/apple-icon-160.png">
+  <link rel="mask-icon" href="<%= App.cdn_origin %>/images/webkit-mask-icon.svg" color="#398df0">
   <link rel="stylesheet" href="<%= main_stylesheet_path %>" data-alt="<%= alternate_stylesheet_path %>">
   <%= javascript_tag 'application', asset_host: false %>
   <%= javascript_tag 'docs' %><% unless App.production? %>