html to markdown occurs locally (clean)

Improves security and privacy by converting html to markdown locally (instead of via an untrusted API).

- Use this PR instead of https://github.com/codeforamerica/oakland_answers/pull/16 (used `cherry-pick` to remove commits that were not germane to this feature)
- Currently requesting to pull against `dev` branch. is this ok? or should it be master?
- Includes a simple spec