simplify the wui

[zooko]

If you are looking at a directory and you want to get a read-only link to a child of it, currently you have to get the read-only URI and then prepend that with http://127.0.0.1:8123/uri/. It would be nice if there were a "read-only-link" hyperlink next to the "URI-link" hyperlink. But for that matter, maybe the time has come to make all representations of caps emitted by tahoe start with http://127.0.0.1:8123/uri/. Then we could simplify the wui to offer the hyperlink to the file and a read-only hyperlink (if and only if you have write access to the file). We could remove or move into a special metadata-page the JSON and text-plain links, and remove entirely the URI and readonly-URI links, the URI link for files, which is identical to the actual hyperlink to the file, and the text-plain link.

If we implement #103 -- "navigate by URIs in webish.py" -- then we can remove the URI link for files, too.

Therefore, we can have a nice simple wui with at most two hyperlinks per child: the read-write link (if appropriate) and the read-only link (if appropriate). In addition to those two links, there would be a set of actions (buttons) available, if appropriate: delete, overwrite, rename, check, view metadata.