[TASK] Make coverletter work using ajax+dropzone

Classes/Controller/Ajax/UploadController.php

Classes/Controller/JoblistController.php

...Property/TypeConverter/UploadedFileReferenceConverter.php

Resources/Private/Language/locallang.xlf

Resources/Private/Templates/Joblist/ApplyForm.html

Resources/Public/JavaScript/Form/upload.js

Resources/Public/JavaScript/sgJobs.js

ext_localconf.php

package-lock.json

package.json