Retire DevDocs's mailing list in favor of Gitter

Author: Thibaut

.github/CONTRIBUTING.md

@@ -27,8 +27,6 @@ Want to contribute? Great. Please review the following guidelines carefully and
 2. Make sure your feature fits DevDocs's [vision](https://github.com/freeCodeCamp/devdocs/blob/master/README.md#vision).
 3. Provide a clear and detailed explanation of the feature and why it's important to add it.
 
-For general feedback and ideas, please use the [mailing list](https://groups.google.com/d/forum/devdocs).
-
 ## Requesting new documentations
 
 Please don't open issues to request new documentations.  
@@ -70,7 +68,7 @@ To ask that an existing documentation be updated, please use the [Trello board](
 Besides new docs and features, here are other ways you can contribute:
 
 * **Improve our copy.** English isn't my first language so if you notice grammatical or usage errors, feel free to submit a pull request — it'll be much appreciated.
-* **Participate in the issue tracker.** Your opinion matters — feel free to add comments to existing issues. You're also welcome to participate to the [mailing list](https://groups.google.com/d/forum/devdocs).
+* **Participate in the issue tracker.** Your opinion matters — feel free to add comments to existing issues.
 
 ## Coding conventions
 
@@ -80,4 +78,4 @@ Besides new docs and features, here are other ways you can contribute:
 
 ## Questions?
 
-If you have any questions, please feel free to ask on the [mailing list](https://groups.google.com/d/forum/devdocs).
+If you have any questions, please feel free to ask them on the contributor chat room on [Gitter](https://gitter.im/FreeCodeCamp/DevDocs).

README.md

@@ -9,7 +9,6 @@ Keep track of development news:
 * Join the contributor chat room on [Gitter](https://gitter.im/FreeCodeCamp/DevDocs)
 * Watch the repository on [GitHub](https://github.com/freeCodeCamp/devdocs/subscription)
 * Follow [@DevDocs](https://twitter.com/DevDocs) on Twitter
-* Join the [mailing list](https://groups.google.com/d/forum/devdocs)
 
 **Table of Contents:** [Quick Start](#quick-start) · [Vision](#vision) · [App](#app) · [Scraper](#scraper) · [Commands](#available-commands) · [Contributing](#contributing) · [License](#copyright--license) · [Questions?](#questions)
 
@@ -145,4 +144,4 @@ I also wish that any documentation file generated using this software be attribu
 
 ## Questions?
 
-If you have any questions, please feel free to ask them on the [mailing list](https://groups.google.com/d/forum/devdocs).
+If you have any questions, please feel free to ask them on the contributor chat room on [Gitter](https://gitter.im/FreeCodeCamp/DevDocs).

assets/javascripts/templates/pages/about_tmpl.coffee

@@ -21,7 +21,7 @@ app.templates.aboutPage = -> """
   <ul>
     <li>Follow <a href="https://twitter.com/DevDocs">@DevDocs</a> on Twitter
     <li>Watch the repository on <a href="https://github.com/freeCodeCamp/devdocs/subscription">GitHub</a>
-    <li>Join the <a href="https://groups.google.com/d/forum/devdocs">mailing list</a>
+    <li>Join the <a href="https://gitter.im/FreeCodeCamp/DevDocs">Gitter</a> chat room
   </ul>
 
   <h2 class="_block-heading" id="copyright">Copyright and License</h2>
@@ -48,7 +48,7 @@ app.templates.aboutPage = -> """
     <dt>Where can I suggest new docs and features?
     <dd>You can suggest and vote for new docs on the <a href="https://trello.com/b/6BmTulfx/devdocs-documentation">Trello board</a>.<br>
         If you have a specific feature request, add it to the <a href="https://github.com/freeCodeCamp/devdocs/issues">issue tracker</a>.<br>
-        Otherwise use the <a href="https://groups.google.com/d/forum/devdocs">mailing list</a>.
+        Otherwise, come talk to us in the <a href="https://gitter.im/FreeCodeCamp/DevDocs">Gitter</a> chat room.
     <dt>Where can I report bugs?
     <dd>In the <a href="https://github.com/freeCodeCamp/devdocs/issues">issue tracker</a>. Thanks!
   </dl>

assets/javascripts/templates/pages/root_tmpl.coffee.erb

@@ -14,7 +14,7 @@ app.templates.intro = """
       <li>Run <code>thor docs:download --installed</code> to update all downloaded documentations.
       <li>To be notified about new versions, don't forget to <a href="https://github.com/freeCodeCamp/devdocs/subscription">watch the repository</a> on GitHub.
       <li>The <a href="https://github.com/freeCodeCamp/devdocs/issues">issue tracker</a> is the preferred channel for bug reports and
-          feature requests. For everything else, use the <a href="https://groups.google.com/d/forum/devdocs">mailing list</a>.
+          feature requests. For everything else, use <a href="https://gitter.im/FreeCodeCamp/DevDocs">Gitter</a>.
       <li>Contributions are welcome. See the <a href="https://github.com/freeCodeCamp/devdocs/blob/master/CONTRIBUTING.md">guidelines</a>.
       <li>DevDocs is licensed under the terms of the Mozilla Public License v2.0. For more information,
           see the <a href="https://github.com/freeCodeCamp/devdocs/blob/master/COPYRIGHT">COPYRIGHT</a> and