Enhancement: Add chunked upload with tus-js-client

Whenever the backend server advertises TUS support, uploading files will
use TUS as well for uploading, which makes it possible to resume failed uploads.
It is also possible to optionally set a chunk size by setting a numeric value
for "uploadChunkSize" in bytes in config.json.

https://github.com/owncloud/web/issues/67
https://github.com/owncloud/web/pull/3345
