Add translators to the credits dialog

- Add tooling at //tools/i18n to generate the file with the information
  about the translators: //src/json/i18n-credits.json.
- Change credits.json to remove an entry from a translator, who is now
  mentioned in the i18n credits.
- Change Makefile and release.bash to incorporate the i18n credit
  generation in the process of building the extension.
- Change options page to accommodate the translators section.

Change-Id: I7f3991f9c456c381832f4a7bebdfc5581ef9e4be
diff --git a/src/options.html b/src/options.html
index 2765d1a..ade625c 100644
--- a/src/options.html
+++ b/src/options.html
@@ -46,8 +46,18 @@
     </div>
   </dialog>
   <dialog id="credits_dialog">
-    <h3 data-i18n="credits"></h3>
-    <div class="content_area">
+    <div class="scrollable">
+      <h3 data-i18n="credits"></h3>
+      <div class="entry">
+        <a href="https://gtranslate.avm99963.com/" class="homepage" target="_blank" data-i18n="credits_homepage"></a>
+        <h4 data-i18n="credits_translations"></h4>
+        <div data-i18n="credits_translations_paragraph">
+        </div>
+        <ul id="translators">
+        </ul>
+      </div>
+      <div class="content_area">
+      </div>
     </div>
     <div class="action_buttons">
       <button id="credits_ok" data-i18n="ok"></button>