Add ability to import and export list items

List item are only unique within the list.

Help topics can now specify one or more additional forms to be included on
the help topic and can also specify the sort order of those forms.
Furthermore, individual fields can be disabled per help topic, so that
unnecessary fields can be omitted when necessary, per help topic.

The disabled flag is recorded along side the field data so that the field
will not be accidentally added to the form later automatically. There is no
interface in this commit to enable a field which was disabled by the help
topic when ticket was created.

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.

Also fix a crash when re-rendering the new ticket by staff page after a
validation error.

Attachment settings are now exclusively handled by the settings on the
ThreadEntry field on the ticket details form. Enabling attachments as well
as tuning attachment validation settings are all handled from the one place
now.

Add an interface for custom list items.
Implement CustomListItem for built-in and custom lists.

Ultimately, in the user information ("contact information") form, forbid
updating a user's email address to an email address in use by another user.
Because email addresses are a unique identifier of user accounts, they
cannot be reused between different users.

Fixup several minor bugs concerning initial experience

Moved to an initial form which specifies the ticket's priority and issue and
changed the rendering to render things properly. Now the user can decide
where priority shows on the client side, and the priority privacy setting is
placed in the dynamic form wizard.

The standard form is added to every ticket without option. Extra forms can
be defined and associated with help topics which can additionally be added
to tickets upon creation. This allows for standardization of the dynamic
data location for searches and filtering.

Implemented advanced search for dynamic data. Along with reinstating the
basic ticket search on keywords

Implemented ticket filtering on dynamic data for both keyword searches as
well as searches for special fields (drop-down lists, etc.)

Phone number for users is now completely optional

This moves client information like name and email address out of the general
dynamic forms data for a ticket. It really paves the way for the first-class
user of the future.

Previously, form sections were grouped into form sets for reusability. This
patch drops the form sets and makes form sections the new "forms".
Eventually a section-header field will be added that technically does not
add any dynamic data to the form, but allows for the same feature as having
a form set with multiple sections.

*This is a major redesign / rework of the osTicket base*

This patch drops the concept of static ticket metadata and allows for an
admin-configurable arbitrary data that is attachable to tickets

The system is architected such that the base osTicket install now comes with
a "default" form that has fields for subject, name, email, and phone number.
This form is editable to allow for the addition of arbitrary other fields;
however, the basic fields must remain in order to be associated with a
help-topic and attached to a ticket.

This concept can be expanded to allow for arbitrary data associated with
registered clients or ticket thread items.

Forms are comprised of sections. Sections have a title and instructions
properties and a list of fields. Fields have various implementations to
represent different data such as text, long answer, phone number, datetime,
yes/no, and selections, and are configurable to define the look and feel and
interpretation of the respective form field.

Dropdown lists are represented as "Dynamic Lists", which are
admin-configurable lists of items. Dropdowns can be optionally represented
as Bootstrap typeahead fields.

This also adds the start of a simple ORM which will hopefully be expanded in
the future to support multiple database platforms. Currently, only MySQL is
implemented.