(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.
I didn't look into if it was handled automatically or not, and don't have much idea how I would go about finding out; again, this is basically copied directly from Edges.pm. :)