Remove social links box (#405)

Remove social links box on main page by:

* Removing reference to our defunct Twitter presence, due to the
  inability of the PMC to properly share the responsibilities of
  maintaining our presence there (TweetDeck is now a paid premium
  service, and we cannot use it any longer for free) and the lack of
  interest from the PMC in maintaining our presence there
* Move Linked in information to the people page
* Remove redundant GitHub link
* Remove unused images and includes pages related to social media

Author: ctubbsii

_config.yml

@@ -13,7 +13,6 @@ description: > # this means to ignore newlines until "url:"
   The Apache Accumulo™ sorted, distributed key/value store is a robust, scalable,
   high performance data storage and retrieval system.
 url: "https://accumulo.apache.org" # the base hostname & protocol for your site
-twitter_username: ApacheAccumulo
 exclude: [vendor]
 latest_release: 2.1.2
 javadoc_version: 2.1.2

_includes/icon-github.html

@@ -84,12 +84,6 @@ code {
   font-size: 12px;
 }
 
-#sociallinks td {
-   /* no lines in the link table */
-   border-top: none;
-   border-bottom: none;
-}
-
 #bannertext {
     margin-top: 10px;
     text-align: left;

_includes/icon-github.svg

@@ -53,11 +53,6 @@ <h4>Stable</h4>
         </div>
       </div>
     </div>
-    <div class="row">
-      <div class="col-sm-12 card mb-4">
-        {% capture social-include %}{% include social.md %}{% endcapture %}{{ social-include | markdownify }}
-      </div>
-    </div>
     <div class="row">
       <div class="col-sm-12">
         <a class="acevent" data-class="img-responsive center-block" data-format="wide" data-mode="light" data-width="240"></a><br/>

_includes/icon-twitter.html

@@ -197,6 +197,15 @@ GitHub also has a [contributor list][github-contributors] based on commits.
 | Wil Selwood         | [Satellite Applications Catapult][SACAT]                          | [GMT][GMT]/[BST][BST] |
 | Xiao Wang           | [Stevens Institute of Technology][SIT]                            | [ET][ET]              |
 
+## More
+
+Many people with a background working with Accumulo can also be found on
+various professional and social networking sites. One such group is the [Apache
+Accumulo Professionals][linkedin-group] group on Linked in. Membership in these
+groups is not in any way an endorsement of their qualifications from either the
+ASF or the Apache Accumulo PMC.
+
+[linkedin-group]: https://www.linkedin.com/groups/4554913/
 [github-contributors]: https://github.com/apache/accumulo/graphs/contributors
 [ANAVATION]: https://www.anavationllc.com
 [ANTHEMENG]: https://www.anthemengineering.com