Rename pdf2htmlEx everywhere in the JS and PHP code to pdfconverter

Please make sure, that the name  can not be found in our code or md-files, that are publicly available on github.

So inside the visual-plug-in it is ok, but not outside of it.

Please make sure, that everything related to it is also removed from git history. For this you need to talk to with Thomas.

Everywhere in those positions pdf2htmlex should be replaced by pdfconverter

I found it in the following files:

https://github.com/translate5/translate5/blob/develop/docs/ERRORCODES.md
 
/application/modules/editor/database/sql-changelog-2020-09-07.sql
/application/modules/editor/Controllers/IndexController.php
 
/docs/CHANGELOG.md

Please also make sure, that neither pdf2htmlex nor pdf2html are contained in the html or JS output of the visual-plugin, that is send to the browser