Newsletters in queue with uid

Hi,
i created this branch to change the way of newsletters are put in queue jobs.
If we store their path, if someone move/rename/delete the content before the dispatch, the newsletter stays in a broken state and the system doesn't give any information about this problem.
Another problem is that with the path, if the content is moved/renamed, it cannot be found with a restrictedTraverse.

With the UID we can handle the case when the content is renamed/removed.
I have also handled the case when the content is deleted, and the info is given to the user and write in the log.

I updated translations and tests.