POST /uri?t=upload should give Upload Results consistently (even mutable)

[warner]

The docs in source:docs/frontends/webapi.rst​ currently claim (in section 6.5, "Browser Operations" > "Uploading a File", around line 1150) that doing a POST to /uri?t=upload will always "return an HTML page that describes the results of the upload. This page will contain information about which storage servers were used for the upload, how long each operation took, etc".

At present, we only actually do that for immutable uploads. When creating or modifying a mutable file, it just returns the filecap.

We should implement a PublishResults object, have mutable-file creation (and probably modification) populate it with a few useful items, and return it from .publish, then write a renderer for it.