Incoming messages which arrive into labels we know during sync are now
presented to the IMAP clients.
We also allow messages to be deleted while syncing if deleted on other
clients.
Other operations such as moving, marking messages as read and label
operations need to be considered in a follow up patch as they are far
more complex.
Rather than having the services subscribe to each individual event type,
have only one subscriber for the whole event object. This spreads the
logic to the service through the `EventHandler` type.
This is important for the next stage where the IMAP service will be
required to apply events from the past.
Handles all IMAP related tasks. Unlike the previous iteration, this new
service automatically adds and removes users from Gluon by interfacing
with server manager.
