added media:thumbnail to the MRSS feed

[abrahammartin]

• Modified the export_sms_feed.sh to use the media image_id or in its absence its collection image_id
• Added a new command line parameter for "base-image-url" that will be used as the base URL where the images are being served (same as "base" for videos)

Closes https://github.com/uisautomation/sms-webapp/issues/36

[rjw57]

Merged and script mentioned in https://wiki.cam.ac.uk/ucs/SMS_-_Streaming_Media_Service#Export_from_SMS_to_jwplayer updated.