Allow thumbnails for non-videos, and render a default thumbnail in Media contexts

This patch combines two feature requests, which could be separated if needed, but since they're both relate to thumbnails and are small, I thought combining into one patch made sense:

1. Allows non-video to have thumbnails. (e.g., Embed.ly's response for SoundCloud identifies the "type" as "rich", but supports a thumbnail_url).

2. When media_oembed module is enabled, adds an extra #pre_render function to display Media's default icon as the thumbnail when no other thumbnail is available. This is necessary for wysiwyg and media browser integration.