Implement front end to LJ::Support::define_cat

- Consolidates changes to table supportcat.

- Adds new priv.

- Basic form content and processing.

Fixes #965
why not use [% form.textbox %]? It's less verbose and also means that if there was an error, and the form is shown again, the submitted form values are used automatically. 

If you need to set default values, you can set them on $vars->{formdata}, if *not* posted to