admindb: new label for 'preserve for admin'

[slyh]

In my experience nobody really knows what the 'Preserve messages for the site administrator' checkbox does and why it is there. -- But after I changed the text to 'train spamfilter' people really started using it and it became useful.

Thus I suggest to find a new label for this checkbox and clearly indicate a spam-related action (so the user knows a marked mail will be used to train a spamfilter).

[slyh (slyh)]

The file mailman-admindb-spam.diff was added: change checkbox 'preserve for admin' to 'mark as spam' in admindb