[ticket/11874] Do not always prepend the web path; only replace phpbb_root_path

PHPBB3-11874
[ticket/11874] Fix tests

PHPBB3-11874
[ticket/11874] Correct when $phpbb_root_path is appended to build_url()

PHPBB3-11874
Merge pull request #1764 from EXreaction/ticket/11874

[ticket/11874] Do not always prepend the web path; only replace phpbb_root_path