Only the admin user (sudo) is allowed to send notifications to other
users. The normal users can only send notifications to themselves.
This is to prevent attackers to craft malicious notifications and send
them to other users using RPC.
Correction based on the idea of @hbrunn
It opens for extensions.
The implemented options now allow to configure the name and the icon of
the link for the action. Addons could easily add new features as the
arguments are no longer predefined, they'll all be available in the
'options' dictionary.