acceptable_aliases: allow deletion

This is one of a set of three commits to mailman-core, mailman-client and postorius in order to fix the broken accepted_aliases settings dialog. This change treats a posted REST request with a list of an empty string as an attempt to remove the acceptable_aliases all together. The problem here is that an empty text-box "Acceptable aliases" normally would not post anything, so mailmanclient posts now (after another patch) an empty string in order to signal that something goes on.