SOLIDPOD: add save of large files to external pod

[jesscmoore]

Description

Currently we are unable to write image files to an external owner's pod.

The parameters for writeLargeFile() should be consistent with writeExternalPod, allowing the provision of the owner's webId. Currently writeLargeFile() always writes to the user's Pod.

Why

So that as a user I can:

1. edit large file: read and edit a large externally owned file
2. add image to a note or news article: where the note or news article is externally owned and I want the image to have the same externally owner (eg. notepod or communitypod respectively).
3. edit image: read an externally owned image, and potentially edit or change the image (not in an app yet).

Closing Criteria

Checklist for closing the issue:

• No errors from make prep
• All tests pass make qtest
• large files can be written to external owned pods

Alternatives

• Use case 2: articles could have image content stored in the pod of the user who adds the image to an existing article without that image. That way each creator would retain ownership of the content they release. Metadata would need to show permissions of the different media components. Sharing would need to share all components for recipients to see the same article.
• Use case 3: updates to an image could generate an image copy owned by the image editor. In practise, the editor may be providing editing as a service and not want to owned the edited image.

[gjwgit]

• As discussed, writeLargeFile() should be able to write to local pod or a pod of another WebID.
• Similarly we should collapse writeExternalPod() into writePod() so that the argument indicates whether local of other webID pod.
• If permission to write to other pod is not present then return an exception.

[tonypioneer]

Large files can now be uploaded to the external POD. The following is the uploading page on demo app.

The following is the content of the folder I uploaded.

/opt/solid/server/tchen/exampleApp/data/external_pod# ls
stu.pdf.ttl