Add token (authtoken) based authentication to User authentication backend.

Monitor failed login attempts and enforce timeouts

  - StaffAuthenticationBackend - handles backends supported in staff panel
  - UserAuthenticationBackend - handles backends supported in user panel

Registering a backend at parent level (AuthenticationBackend) will make it
available to both panels.

This is necessary to force downstream classes to implement key required
functions.

Add variable replacer callback

is_callable will report ANY function as available

This is necessary because the system depends ticket priority to sort and
display ticket data.

Allow autocomplete on email address

Reviewed-By: Peter Rotich <peter@osticket.com>

Show extra forms for help topic first

Reviewed-By: Peter Rotich <peter@osticket.com>

Export custom fields with the ticket queue export

Reviewed-By: Peter Rotich <peter@osticket.com>

Fix subject ticket number matcher (again)

Reviewed-By: Peter Rotich <peter@osticket.com>

Use case-insensitive matching on email address

Reviewed-By: Peter Rotich <peter@osticket.com>

Collaborators

Reviewed-By: Jared Hancock <jared@osticket.com>

Provide facility to set collaborator status on add.

This patch addresses an issue where a client may have upper-case letters in
their email address. When visiting the client portal to check the ticket
status, previously, the exact same case would be required in the 'Email
Address' box.

This patch remove the case sensitivity for email logins.

No longer require a priority for help topics

Reviewed-By: Peter Rotich <peter@osticket.com>

Pre-select default priority

Reviewed-By: Peter Rotich <peter@osticket.com>

Reviewed-By: Jared Hancock <jared@osticket.com>

Extra forms associated with the help topic are now shown above the ticket
details form on both the staff and client interfaces.

Also add in the user email address after the user name.

This patch also adds an ::export() method to the FormField() class which
allows a field to define how the data should be formatted when exported.