Refactor upload business logic and update tus-js-client to 2.3.0

[kulmann]

We're using tus-js-client in version 1.8.0. The client is now at version 2.3.0 and received major improvements regarding upload tracking and fingerprinting. We should look into upgrading the client.

In addition to this it makes sense to clean up our code base regarding uploads. At the moment most of the code lives in the huge mixins.js of the files app. It makes sense to move code into helpers dedicated to uploads.

This is also a good chance to have a look at some earlier ideas of improving the upload handling with TUS:

• Add TUS creation-with-upload support #3436
• WIP Improve TUS error handling #3472

[pascalwengerter]

Link to migration guide: https://tus.io/blog/2020/05/04/tus-js-client-200.html

[pascalwengerter]

Updating the tus-js-client was done in #6177 and the refactoring of the upload logic will be done together with #6268 so closing this ticket