Create react component: CommentForm

[patcon]

This is exploratory and quite low priority, but might be neat to try to extract more components into react, and maybe see how to swap out pieces of backbone (which these views are currently written in).

• should use same theming library that existing vis2 components use: theme-ui
• for now, is just about creating a frontend component that could feasibly be swapped in, not about actually wiring it in yet (which will require more thought)

https://github.com/compdemocracy/polis/blob/edge/client-participation/js/views/comment-form.js

[patcon]

Prior art: https://github.com/UT-HAI/polis/tree/HAI-dev/client-deliberation/src/components