Closes #6664 - Allow override login form from theme

Closes #6664 (closed)

Change

This change allows users to override the theme templates by creating these template files:

• ispconfig/web/themes/my-theme
  • templates
    • login
      • index.htm
      • otp.htm
      • password_reset.htm

This was not possible before, because the _fileSearch method in tpl.inc.php did not consider login pages to be a module. The change I made corrects this behaviour by hard-coding a check to see if the $_SERVER['PHP_SELF'] (currently executing script) starts with /login/. This catches all three login pages that have templates + any future ones.

Testing

• The default theme continues to work without any problems
  • Tested setting the theme to and from default from the Tools > User Settings menu
  • Tested setting the theme in the interface/lib/config.inc.php
  • Default index.htm works normally
  • Default password_reset.htm works normally
  • Default otp.htm should work normally, but I haven't been able to test this
• A custom theme can override the templates
  • Overriding index.htm works
  • Overriding password_reset.htm works
  • Unsure if otp.htm works, it should, but haven't been able to test it
  • If a template isn't overridden in a custom theme, it falls back correctly (tested on index and password_reset)
  • Tested setting theme through User Settings, works fine
  • Tested setting theme in interface/lib/config.inc.php
• I've clicked through the modules and can't find any broken pages. Everything seems in order
• Impersonating a user works fine, even if they have another theme configured, it switches without any issues

Stability/risk

I don't expect this change to cause any issues for users, seeing how the _fileSearch method falls back to it's original behaviour if the configured theme doesn't override it.

Though do take note of how I implemented the fix: I check if the executing file starts with /login/. I did this intentionally, so it would match the existing pages, but also any future templates in the login directory. Looking at login/logout.php and login/login_as.php I see some hard-coded HTML that may be worth separating into a template in the future.

Code styling

My VSCode removes indentation on empty lines when I save. This has caused a few minor formatting changes in the changed file. I see this as positive, since now the file is now consistent in that empty lines are actually empty.