change from using tag uris to http urls for identifiers

change from using tag uris to http urls for identifiers

Weirdly, I got in an argument with Tim Berners-Lee in #swig about the
tag URIs I was using in FOAF documents. Eventually, I was convinced
that it's a better thing to use HTTP URLs instead. So, now we have
HTTP URLs.

The tricky thing was for users. Since they can change their names, we
can't use their profile URL, since it includes the name. Instead, I
made up a new action, which simply redirects from a user ID to their
current profile URL. This should be sufficiently long-term.

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