Enhancement: Use TUS settings from capabilities

The TUS settings advertise the maximum chunk size, so we now use the smallest chunk size from
the one configured in config.json and the one from the capabilities.

If the capabilities report that one should use the X-HTTP-Override-Method header,
the upload will now use a POST request for uploads with that header set
instead of PATCH.

https://github.com/owncloud/ocis-reva/issues/177
https://github.com/owncloud/web/pull/3568
