[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/8309] Renaming "Last visit" to "Last activity"

PHPBB3-8309
[ticket/8309] Rename $last_visit to $last_activity

PHPBB3-8309
[ticket/11169] Move module 'prune users' to users category

PHPBB3-11169
[ticket/8309] Moving key to get keys in alphabetical order

PHPBB3-8309
[ticket/10590] Remove confirmation page after posting

PHPBB3-10590
[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/8309] Removing key with similar function to avoid redundancy

PHPBB3-8309
[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
[ticket/10590] Fix functional tests are post confirm message removal

PHPBB3-10590
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.
[ticket/12198] Language issue in en/common.php

Deleted lines as they are not needed anymore.

PHPBB3-12198
Merge branch 'nickvergessen/ticket/elsensee/10590' into ticket/10590
[ticket/12285] Fix mult-lingual issues between extensions

PHPBB3-12285
[ticket/11766] Hide edit, quote and delete buttons if topic is locked

But show quote button to guests if topic isn't locked

PHPBB3-11766