Connection and server status

There was no way to check the connection between the client and the server or between the client and external links related to KA Lite (Wiki and FAQ).

- Now there's the possibility to check the local server status and the central server status, from the client. 
 
- If the local server is offline, the admin user, will be able only to delete videos. The download videos/subtitles will be disabled.

- The easy admin page will only show external options if the client can connect to the internet.

Files edited:
- api_urls.py
- api_views.py
- khan-lite.js
- admin_distributed.html
- video_download.html
- decorators.py