Fix array to string conversion error.
[ticket/63831] Fix missing "Unsubscribe from x" string.
[ticket/63856] Check that the PHPBB_USE_BOARD_URL_PATH isn't already defined.
[ticket/63881] Do not rely on search submit button to check if form was submitted.
Merge branch 'master' into phpbb-com
Merge branch 'phpbb-com' into develop