delete.php: it seems that assign an array element to a string variable does not convert the string to an array. CR thought this was the case in the past (maybe a change in PHP7.2)

Html2Pdf.php: remove check if wkhtmltopdf is executable - without the check, it's possible to specify variables, like LD_LIBRARY_PATH, in front of the executable. The latest development version of wkhtmltopdf needs specific libssl and libcrypt libs.
Manual.rst: add a note the the latest wkhtmltopdf can be requested from the QFQ team.

ext_conf_template.txt: remove default for 'baseUrl'. Change dateFormat to 'dd.mm.yyyy'. Add missing default values for cssClassQfqFormPill and cssClassQfqFormBody.

Database.php, DatabaseUpdate.php: implement 'specificMessage' in prepareExecute().