1.17.1: PHP8 threw an error because of an undeclared
        const variable "UNKNOWN". changed both 
	     appearances to the string value 'UNKNOWN'

1.17: Reset message for 404 error handling which is now
      overriden

1.16: Iconfont a11y fix (yellowled)

1.15:

    * Load lang with lang api
    * Add Serendpity 2.0+ backend markup
    * Fix inconistencies

1.14.1: Use serendipity_specialchars

1.14: Added stricter array check