Signed-off-by: Pierre Mazière <pierre.maziere@gmx.com>

Signed-off-by: Pierre Mazière <pierre.maziere@gmx.com>

Signed-off-by: Pierre Mazière <pierre.maziere@gmx.com>

There is now a "latest" collection (or "latest_nsfw").
The feature didn't look much used, since it didn't work on many
backends.
Using collections will make it easy to support other things
like most viewed, featured, etc.
As a bonus, I added tests for every backend with the feature.

do not use thumbnail_url anymore (closes #810), use set_empty_fields() method, do not give fields in constructor of BaseVideo anymore