Add drag and drop feature to upload zip to server

[BTWS2]

[BTWS2]

Clicking on "Upload a file", results in a warning: File is required.

upload.mp4

[rien]

This warning is intentional - a ZIP-file is required to start an analysis.

We can add support for drag-and-drop while implementing #1121.

[BTWS2]

Thank you for considering the drag-and-drop feature.

My minor remark was that there is a warning when the user is still busy with selecting the file. I would expect the warning is only shown when continuing the form by typing a file name, choosing a programming language or pressing the Analyse button without uploading a file (which isn't possible because it's grayed out).