Implement front end to LJ::Support::define_cat

- Consolidates changes to table supportcat.

- Adds new priv.

- Basic form content and processing.

Fixes #965

Hmm, right. I think that's all right, though; labels are usually on the
right when it comes to checkboxes and the like, so it fits expectations.

if you're worried about this making things looking disorganized you
might try using a fieldset around the checkboxes to group them. But I'd
consider that optional, not a must.