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ARROW-2504: [Website] Add ApacheCon NA link

Place a 234x60 link in the navbar next to the Apache Software Foundation link.

Screenshot for full width:
![image](https://user-images.githubusercontent.com/7432951/48995065-1213f700-f10c-11e8-944f-e5d26f1bfe8b.png)

Screenshot for medium width:
![image](https://user-images.githubusercontent.com/7432951/48995076-1b04c880-f10c-11e8-8abf-a3d2ef204596.png)

Disappears in small width -- screenshot:
![image](https://user-images.githubusercontent.com/7432951/48995108-31128900-f10c-11e8-96a7-066e377081fa.png)

Author: Tanya Schlusser <tanya@tickel.net>

Closes #3030 from tanyaschlusser/master and squashes the following commits:

77c6cd323 <Tanya Schlusser> ARROW-2504:  Add ApacheCon NA link