Tests: revise StringTest.php

- Add assertion to check string_sanitize_url() when $g_short_path = '/'
  This is a bit of a hack, but it gets the job done
- Add test case for login page URL redirection issue #17648
Fix URL redirection issue in login_page.php

When Mantis is installed at the web server's root, $g_short_path is set
to '/'. string_sanitize_url() removes the trailing '/' from the short
path, which causes the URL to be incorrectly categorized as "type 2",
thus allowing cross-site redirection to occur.

By making checking that the short path is not empty before setting URL
as type 2, we ensure that we categorize it as type 3, which then forces
the function's return value to 'index.php'

Fixes #17648 (CVE-2014-6316)
Update mailmap file
Fix #17827: PHP STRICT warnings in Disposable library
Update CREDITS
Fix invalid link in 'nosniff' header comment

Missed that occurence in html_api.php when I updated it in
file_download.php (e66ecc9f886a97a3b54a8bdaa9afbfa6902f1d9a).
Bump version and update release notes for 1.2.18
Changed version to 1.2.19dev [skip ci]
In feedback state, change back to previous status

When an issue is in $g_bug_feedback_status, $g_reassign_on_feedback
is ON and the reporter adds a note, the issue's status is changed back
to what it was before being feedback, unless it was equal or greater
than $g_bug_resolved_status_threshold, in which case the status is set
to $g_bug_submit_status if the issue has a handler or
$g_bug_assigned_status status otherwise.

Fixes #4469
Align to coding guidelines and add PHPdoc for missing param
Fix whitespace
WIP: allow non-reporters to provide feedback on issues

TODO: split commit in cleanup vs function; test; bug ref

Fixes #