Include method in thumbnail media name (#7124)

This fixes an issue where different methods (crop/scale) overwrite each other.

This first tries the new path. If that fails and we are looking for a
remote thumbnail, it tries the old path. If that still isn't found, it
continues as normal.

This should probably be removed in the future, after some of the newer
thumbnails were generated with the new path on most deployments. Then
the overhead should be minimal if the other thumbnails need to be
regenerated.

Signed-off-by: Nicolas Werner <nicolas.werner@hotmail.de>