Get thumbnails for videos

Support thumbnails for Videos …
* Add VideoEmbed::thumbnail() and rework existing embed() code
* Make reports::save_media() save video thumbnails too.
* Allow grabbing video embeds without echo'ing immediately.
* Add events to allow plugins to add additional services
* Make sure video embeds work in report detail view.

This means we have a thumbnail and medium size image available for videos. This is used by the reports listing but could be used in other places in the theme (see TEDx Map)

Closes #294