Move collaboration implementation to thread level

Add ability to extend Form
Support the concept of form ID
Clear/hide field errors on blur

Add a tab for tasks
Add mass action for tasks

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

Key permissions definition array
Move canned and faq permissions to their respective classes.

Allow for permissions to be registered dynamically. This will
allow for the ability for models and plugins to register permissions.

Add ability to add tasks to tickets.

Introduce the concept of tasks. Tasks are to do list that can be attached to
any object e.g Ticket.