Fix regression in test_langs.php

Commit 78cee358 added file Web.config to plugins and lang folder.
test_langs.php treated the file as a plugin folder and a language file.
remove most references to (unused) document_files_prefix config setting
Fix moving of attachments to allow both directions (db<>disk)

This basically builds on the work Damien did before.

In addition, tidy up the file api for later merging into the new db layer
and fix support (;() for oracle in the soap api for file handling.
Merge branch 'fix13-test_langs' of github.com:atrol/mantisbt into atrol-fix13-test_langs
Fix #17510: regression in test_langs.php
Fix: 0017383: Move sponsorship details url to a config setting