This makes it possible for Zulip administrators to delete messages.
This is primarily intended for use in deleting early test messages,
but it can solve other problems as well.
Later we'll want to play with the permissions model for this, but for
now, the goal is just to integrate the feature.
Note that it saves the deleted messages for some time using the same
approach as Zulip's message retention policy feature.
Fixes#135.
K.Kanakhin