(Issue #752) Add Interests.pm file

No real changes made yet, but this helps keep track of what I've done
with the file. Based on Edges.pm.
(Issue #752) Implement interests output

/data/interests outputs interests now!
(Issue #752) Remove interestdata.bml

This file is no longer necessary. The ?int= parameter has no equivalent,
but this was rarely used in any case.
(Issue #752) Stylistic fixes, add HTTP_GONE

A few stylistic fixes made after feedback, and replacing numeric HTTP
codes with their DW::Request equivalent. I had to add HTTP_GONE as it
wasn't already in there.
Merge pull request #757 from Sophira/data-interests

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