Allow the use of form `description` fields in form templates

Right now, we're using `deform` widely on the `h` service for rendering forms. `colander` schemas, which are used by `deform`, have a `description` attribute available. But `h` forms only make use of the `hint` attribute, which gets rendered behind a tooltip.

I believe that tooltips can be appropriate in certain situations, but in other places they introduce extra work for the user and are not accessibility- or (very) mobile-friendly.

I'd like to be able to use and render `descriptions`—right now just in admin forms—by adding `description` property output to the form-field templates.

I've done a _quick_ grep of the codebase and I don't see any use of `description` in `colander` schemas outside of my own new admin schema for groups; thus I'm hoping that exposing `description` in templates would not affect any existing forms (especially user-facing ones) but I'd like to discuss.

The actual work here would be relatively quick: add `description` to several form-field templates and appropriate styling.