(Issue #752) Add user-side /data/interests (JSON), remove site-side /misc/interestdata

Add a JSON API endpoint for retrieving user interests, and remove the old script.

No replacement is being made for the previous ?int= functionality to find the count for a single interest; this was apparently rarely used.

The new file is based on the Edges.pm file, so I've left the credits for that file as I'm not sure how much of the stuff I deleted is theirs. (Commit b879927 shows the exact amount that was deleted.)

Fixes #752.
Well, I guess I could do that, sure. I'd need to separate out the actual data generation into another sub, but that might be a good idea anyway, since that way we can reduce the cope duplication in Edges.pm and Interests.pm, which might also make it easier to do user data controllers in the future.

Probably worth doing that in another pull request though, I think, unless you want to expand the scope of this one?