[ticket/11626] Create get_acp_template method for auth providers

PHPBB3-11626
[ticket/11626] LDAP Auth ACP Template File

PHPBB3-11626
[ticket/11626] Include the template file in acp_board

PHPBB3-11626
[ticket/11626] Change interface to match functionality

Changes the interface so that it matches the new functionality of
phpbb_provider_auth_interface::acp().

PHPBB3-11626
[ticket/11626] Call method only one time per provider

PHPBB3-11626
[ticket/11626] Change the identifier template file in the template

PHPBB3-11626
[ticket/11626] Make identifier uppercase per style requirements

PHPBB3-11626