[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
[ticket/12278] Add mcp.php core event to allow to modify modules display option

This event allows extensions to set display option for custom MCP modules
for extensions which add custom MCP modules
and some of them should be displayed conditionally.

PHPBB3-12278
[ticket/12284] Improve readability of extension manager details

Use span tags like the rest of the ACP does, instead of P tags.
There is no semantic need to use P tags here. These are not
paragraphs.

PHPBB3-12284
Merge remote-tracking branch 'VSEphpbb/ticket/12284' into develop

* VSEphpbb/ticket/12284:
  [ticket/12284] Improve readability of extension manager details
[ticket/12284] Fix functional test for extension manager changes

PHPBB3-12284
Merge remote-tracking branch 'VSEphpbb/ticket/12284' into develop

* VSEphpbb/ticket/12284:
  [ticket/12284] Fix functional test for extension manager changes
Merge remote-tracking branch 'rxu3/ticket/12278' into develop

* rxu3/ticket/12278:
  [ticket/12278] Add mcp.php core event to allow to modify modules display option
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.