add tag {{term-slug}} to be used in templates

I had to move the input out of the label, so I needed an ID to reference the label to. Therefore I added a new tag that uses the slug of the category as identifier.
If you have a term slug `logo`, then you'll likely end up with two elements with the same ID, which is a violation of the HTML spec.

I think the ID should be prefixed with something like `term-`, to prevent such conflicts.