Bug 1329003: improve diff experience

* Use diff-match-patch options for better readability
* Hide diff by default and use toggle to turn it on and off

Author: BychekRU

pontoon/base/static/css/style.css

@@ -1184,7 +1184,7 @@ body > .container {
   text-align: left;
 }
 
-#helpers > section ul li > p.translation-raw {
+#helpers > section ul li > p.translation {
   display: none;
 }
 
@@ -1195,12 +1195,12 @@ body > .container {
 
 #helpers > section ul li > p ins {
   background: #4B6259;
-  color: #7BC876;
+  color: #9CD699;
 }
 
 #helpers > section ul li > p del {
   background: #674B54;
-  color: #FE7777;
+  color: #FE8F8F;
 }
 
 #helpers > section ul li > p ins mark,

pontoon/base/static/css/translate.css

@@ -1708,6 +1708,14 @@ p#sign-in-required > a#sidebar-signin {
   float: left;
 }
 
+#helpers > section ul li > header .toggle-diff {
+  margin-right: 6px;
+}
+
+#helpers > section ul li > header .toggle-diff.active {
+  color: #7BC876;
+}
+
 #helpers > section ul li > header .toolbar {
   float: right;
 }