Implements #12085. Add doc and custom error message. Finalize directDownload notation. Add unit test.

WIP: Implements #12085 Persistent download links. 1) Doku fehlt, 2) Nur eine SQL in der Config ist vermutlich viel zu wenig (es fehlt ein Konzept), 3) AS _link Class sollte diese Modus auch unterstuetzen, 4) symlink dl.php muss aktuell noch manuell angelegt werden.

Fixes #11509. Implement a) detection of broken 'split PDF to JPEG', b) fire 'pdf repair'. Also implement configuration von 'convert' and 'pdf2svg'

B7634. Fixes #7634. Session Timeout zu kurz. The bug was probably not a bug, but a too short timeout together with an misleading exception (logout and in - but reload the page also leads to valid page). The fix is 'take the system default (php.ini) and set that one'.

Refs #7743. Move error mesages to Constants.php. Unit tests use those constants now. 'data-pattern-error' only delivered if a 'pattern' is given. 'required' attribute only delivered is set.

Fixes #7620. Label align left/center/right. Defined by config/form/formelement. FormEditor slightly redesigned.

Refactor: Manual.rst update config variables (reorder), add 'qfqLog'. Support.php: formSubmitLog hardcoded to fileadmin/protected/log. DOCUMENTATION_QFQ > SYSTEM_DOCUMENTATION_QFQ. Remove config var 'logDir'.

Fixes #7407: MariaDB / Ubuntu 18 complains about missing values if column of type TEXT isn't explicit specified in INSERT. New default for database.init=SET sql_mode = "NO_ENGINE_SUBSTITUTION"

F4906 Php Session Timeout: can be specified globally in configuration and per form. Affects only logged in FE User.