Release 1.2.8

* increased session security by activating HSTS-header
* updated russian translation; thx to bykidi
* secured from pishing for all external links due hiding the referrer
* improved some code to reduce notices/warnings on stats page
- fixed XSS vulnerability with TS3 Client name; thx to FarisDev
- fixed changing language on install.php were not working
- fixed error with donut chart on stats page, when the database still have not enough different values for all stats