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 is necessary to allow objects to register data views on the fly.

This allows for specifying that attachments from something like a new
message should be brokered with autoresponses and alerts.

This is a temporary patch for v1.9.7 -- develop-next already has a better
implementation.

Add ability to import and export list items

Also add resend only option (for the case when "Edit and Resend" is not
allowed).

This caches the explosion of the foreign key constraints, which eliminates
the need to unnecessarily explode the foreign key constraint again and again
while building queries. This represents about a 15% speed increase building
large queries.

Add in the ability for an agent to resend a response. Optionally editing the
response before sending it, and setting the signature as is possible with
the usual responses.

When the response is resent, the edited version is marked as GUARDED, and
subsequent edits will result in new links in the history chain. That is,
when a response is edited and resent by an agent, that response will remain
in the history chain.

The new role would like be assigned to power users and would all such users
access to edit any post by any user.

Agents can always edit their own posts, and department managers can edit all
posts while the ticket is in the department they manage.

Threads can be edited by marking the original as hidden and setting it's PID to
the id of the new entry. The new entry has cloned data from the original and
sets the `updated` timestamp to reflect the time of last edit. An edited flag
is added to the new entry to reflect its origin.

This patch suggests that agents can edit their own posts, department managers
can edit posts while the ticket is in their department, and that help desk
admins can edit anything.

If a post is edited more than once, only the most recent edit is kept.

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

This is necessary for some mail clients which indicate encoding in GB2312
but actually use GBK encoding.

This correctly handles a bounce message with the following layout:

multipart/report; report-type=delivery-status
    multipart/alternative; differences=Content-Type
        text/plain; charset="us-ascii"
        text/html; charset="us-ascii"
    message/delivery-status
    message/rfc822

Fixes #1854