Add print support for clients

Feature :: Greater flexibility for fields

This will be used for greater flexibility in visibility and requirement
settings.

This will disable the ability for a user to close a ticket when they
have not filled out all of the required fields. Explains the problem to
the user trying to close and directs them to the edit page. Added some
functions to the $ticket class to make it easy to determine the missing
required fields.

Allow fields to be configured for view / edit / required for both agents and
end users. Fields can also be disabled now so that the field remains in the
form but is no longer displayed for new entries.

Allow tickets to be created without a subject — use the help topic full name
instead.

This is necessary so we can consolidate various settings and permissions
into one bit mask field.

Feature :: Advanced Search 2.0

This significantly reduces the time necessary to render the ticket queue
page with the ORM.

And manage it with updates to tickets. This will ease the sorting and
display of tickets based on the time of last update in the ticket queues

This will contain the effective due date of the ticket — either the due date
or the estimated overdue time of the ticket based on the SLA settings. It
will be maintained with ticket updates.

orm: Add greater support for expressions in order_by

Also re-implement the "exclude answered tickets" and "exclude assigned
tickets" from open queue options.

Define a static ::getQuery() method and define 'view'=>true in the model
meta data and the SQL code retrieved from getQuery will be used inline when
joining the model rather than using a table.

QuerySet::extra() — allow for extra tables and where clauses initially, long
term, support extra fields in the SELECT and ORDER_BY clauses.

QuerySet::distinct() — allow for specifying distinct columns in the query

If a common model is used in more than one select_related model (user and
user__email), avoid fetching the columns for the user model more than once.

This adds support for things like

    SqlInterval::MINUTE(3)
    SqlExpression::minus(SqlFunction::NOW(), SqlInterval::DAY(1))

When using select_related with nullable relationships, ensure that when
constructing models, that a NULL object is retrieved when handling a null
relationship.