Move collaboration implementation to thread level

Preserve text formatting on the internal note posted to a new ticket by
staff if HTML thread is disabled.

When the user is not identified by the web interface, such as processing
email to tickets, attempt to locate and include the user filter information
when matching the filter to the new ticket data.

Unassign tickets on transfer when the target department has assignment
restriction and the assigned staff is not a member.

Disable claim (quick self-assignment) when above restriction is in effect.

Add initial concept of activity alert/subscription model to ticket activity.

When sending alerts to agents, consider the setting of the new message alert
configuration.

Reset field content when building a new form

Fixes #1736 - tickets now have their overdue status cleared when it is closed. It uses the inbuilt clearOverdue method to ensure it is done properly, rather than setting isoverdue=0

This patch includes a slight database migration, and adjusts the
functionality of a few core components.

  * Move collaborators from the ticket to the thread.
    This concept allows collaborators on any object which has a thread,
    including tasks.

  * Add flags to the thread entry
    This will allow flagging thread entries for different purposes.
    Initially this can be used to flag the original message of a thread in
    case a ticket / thread is created without an initial message.

  * Lock becomes more of a utility
    The lock is now disconnected from the ticket and is a separate utility.
    Separately, the ticket and task objects can have a reference to a lock
    object. Furthermore, when submitting some activities to tickets, the
    lock is verified to be owned by the respective agent, and the lock code
    must match a current lock code. The code is rotated on each acquire()
    call to guard against double submissions.

  * Collaborator is an ORM model
    The TicketUser class is broken up now so that the collaborator instance
    can exist apart from a ticket. Email message ids are now generated for
    collaborators without respect for a ticket so that collaborators can be
    properly supported on any thread.

This would be required for new ticket auto-responses to tickets created
without an initial message.

Since tickets can be created without a message now, the emailing system
should also identify the thread the message is associated with, so that a
returning email can be associated with the ticket or task's thread in the
event that it was created without an initial message.

Also try harder to send a relevant In-Reply-To and References header back
to the client with the email message.

This patch suggests a change to the message-id creation process that
includes stamping the receiving user-id (staff or client) along with the
thread-id of the originating notice. This allows detection of threading if
the clinically brain-dead mail client drops all the other header detection
mechanisms, including the tag placed in the email body, on response.

This patch works for both client and agent communication.

This patch converts the central file and threading classes over to use the
ORM.