(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'm not altogether sure how happy I am with relying on Apache2::Const here and plain numbers in DW::Request::Base, even though these numbers aren't going to change. If they're all in ::Base anyway, though, should we be getting rid of all of these subs in ::Apache2, since they serve no additional purpose?