community: use hosted Slack Icon

Use hosted Slack Icon file. instead of externally loaded.
The Icon wasn't properly displayed.

Author: piotrnarajowski

custom-theme/community.html

@@ -19,7 +19,7 @@
                     <p text-center><a href="mailto:dev-unsubscribe@mynewt.apache.org">Unsubscribe</a></p>
                   <p> <a href="https://mail-archives.apache.org/mod_mbox/mynewt-dev/" target="_blank">Mail archives</a> </p>
                   <p> Find us on Slack:</p>
-                  <p> <a href="{{ config.extra.slack_url }}"><img src="https://www.countit.com/images/add_to_slack.png" alt="Slack Icon" title="Join our Slack Community" /></a></p>
+                  <p> <a href="{{ config.extra.slack_url }}"><img src="/img/add_to_slack.png" alt="Slack Icon" title="Join our Slack Community" /></a></p>
                 </div>
         </div>
         <div class="col-md-4 mailing-list">
@@ -56,11 +56,11 @@ <h3>Getting involved</h3>
 
                 <p> If you want to browse the code, <a href="https://github.com/apache/mynewt-core">Mynewt Core Repo</a> is the best place to start.
 
-                <p> If you wish to contribute code, a quick look at the <a href="https://cwiki.apache.org/confluence/display/MYNEWT/Contributing+to+Apache+Mynewt"> recommended steps </a> should help.  
+                <p> If you wish to contribute code, a quick look at the <a href="https://cwiki.apache.org/confluence/display/MYNEWT/Contributing+to+Apache+Mynewt"> recommended steps </a> should help.
 
                 <h3>Bug Submission and Issue Tracking</h3>
                 <p> Issues, features, improvements, and wish-lists are now tracked on the <a href="https://github.com/apache/mynewt-core/issues">Mynewt Core Github Repo</a>. New issues will be marked with the appropriate label by a committer who will review them. </p>
-              
+
                 <p> If you are a contributor and wish to work on a change, it's always a good idea to introduce the change in the dev@ mailing list before submitting a pull request into the GitHub mirror for your change. </p>
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