Enhancement: Resumable uploads

We've implemented Uppy as a library for handling uploads. This concludes the following features and changes:

- Resumable uploads when the backend supports the Tus-protocol
- A nice looking overview for all files that have been uploaded successfully or failed to upload
- Navigation across Web while uploads are in progress
- Improved rendering of uploadProgress-visualization
- Removed `vue2-dropzone` and `vue-drag-drop` libraries

https://github.com/owncloud/web/pull/6202
https://github.com/owncloud/web/issues/5031
https://github.com/owncloud/web/issues/6268
