URI queries should return results for equivalent URIs

[btrask]

One file can have many different URIs, under different algorithms or different encodings (at least until we add #15).

When you query for e.g. target=[uri], we should probably find all of the "synonyms" by default. Or if you search for link=[uri], we should find everything that links to that file, regardless of what URI it uses.

I think this depends on #44. Not entirely sure.

[btrask]

Synonym support has improved a lot. One simplification is that we resolve the URI to one file (the oldest one), and then get the synonyms for that.

There's still more work to do though.