Fix SQL error when deleting user

The initialization of $t_user_table must take place outside of the if
statement, since the variable is used a second time at end of the
function.

Regression introduced by a80f447687722b52160d00c96c21a5be5f4329e1.
Add user to project when locking themselves out

Automatically add the current user with their access level to the
project's user list if after updating it they can no longer access it
(i.e. by making it private).

Fixes #16554
Fix HTML on subprojects div in manage_proj_edit_page.php
Use extract() function instead of manual assignment
Signup page: reformat code for better readability
Captcha: streamline links to securimage library
Captcha: implement refresh functionality

Modified signup page, added jQuery and CSS to allow users to generate a
new captcha in case the previous one is not readable.

Fixes #16567
Add .htaccess file in library/securimage

PHP files in this directory must be accessible, as they are referenced
from the signup page. Without this, the feature may not work under
Apache depending on how it's configured.

Fixes #16566