fix(security): Prevent XSS with warning messages (`addWarningMsg()`) (refs #4872)

Also:
- Prevent XSS with action done message (`addActionDone()`)
- Prevent XSS with error message (`exitError()`)
- Add `cleanhtmljs()` function in `freedom_util.php`
fix(exitError): Add missing include of freedom_util.php (refs #4872)

Include "FDL/freedom_util.php" for proper definition of cleanhtmljs()
function.
Merge pull request #1218 from eguaj/3.3-A4872

fix(security): Prevent XSS with warning messages (`addWarningMsg()`) (closes #4872)