[ticket/12160] Add function to check if phpBB is installed.

PHPBB3-12160
[ticket/12160] Check if phpBB is installed before creating phpBB container.

PHPBB3-12160
[ticket/12160] Move phpbb_check_installation_exists() to functions_install.php

PHPBB3-12160
[ticket/12160] Use phpbb_check_installation_exists() in the other modules.

PHPBB3-12160
Merge remote-tracking branch 'prototech/ticket/12160' into develop

* prototech/ticket/12160:
  [ticket/12160] Use phpbb_check_installation_exists() in the other modules.
  [ticket/12160] Move phpbb_check_installation_exists() to functions_install.php
  [ticket/12160] Check if phpBB is installed before creating phpBB container.
  [ticket/12160] Add function to check if phpBB is installed.