Add default presentation

Polls can only be used when there is a presentation, therefore
I think that having the default presentation available can be
useful.

Note that the current nginx configuration in this repository
does not specify where http://DOMAIN/default.pdf actually
resides, so nginx's
[default](http://nginx.org/en/docs/http/ngx_http_core_module.html#root)
`root html` is used.

docker-compose.yml

@@ -46,6 +46,7 @@ services:
       - bigbluebutton:/var/bigbluebutton
       - ./mod/nginx/bbb:/etc/nginx/bbb
       - ./mod/nginx/bigbluebutton:/etc/nginx/conf.d/default.conf
+      - ${DEFAULT_PRESENTATION}:/etc/nginx/html/default.pdf
     networks:
       - bbb-net
     extra_hosts:

sample.env

@@ -21,7 +21,7 @@ WELCOME_FOOTER=This server is running <a href="http://docs.bigbluebutton.org/" t
 # use following line for an additional SIP dial-in message
 #WELCOME_FOOTER=This server is running <a href="http://docs.bigbluebutton.org/" target="_blank"><u>BigBlueButton</u></a>. <br><br>To join this meeting by phone, dial:<br>  INSERT_YOUR_PHONE_NUMBER_HERE<br>Then enter %%CONFNUM%% as the conference PIN number.
 
-
+DEFAULT_PRESENTATION=./conf/default.pdf
 
 
 # -------------------------------