refactored the way thumbnail's are handled slighly. Now, if a cached
copy of a thumbnail exist, that URL will be used directly without
having to redirect through thumbnail/get. The controller action is only
used to create the initial cached copy, after that the cached copy will
be used.
renamed "thumbnailUrl" to "thumbnail", removed unnecessary if-clause
from _gridItem (we will always pass an object now)
removed type constants from the thumbnail class