The DirectMessage plugin allows users to send Direct Messages

Installation
============
This plugin is enabled by default

Settings
========
none

Changes from previous release
=============================

- Migrate from message table to notice table

This change implied the write of upgrading logic, the addition of a new
Notice scope (NOTICE::MESSAGE_SCOPE) and updating the save logic.

- Support Federation

DM is still in charge of local communications-only but it now uses a few new
custom events to allow remote handling of the private messages.

TODO
====

- Review API actions, broken after new update
- Review Command events
- Update messagelistitem (UI) to support multi-recipient. Right now we present only
one of the recipients in the message header.
- Update messagelistitem (UI) to support no-recipient, which happens when a message
is sent to profiles that blocked the sender. Right now we don't present this messages
at all because of the UI requirements, but it is still stored in the database.
- Add delete, like and reply actions. Replies need further changes like adding
support for private-conversations.