Fix markdown in iteration descriptions

What does this MR do and why?

Related to #352279 (closed)

Render iteration description markdown correctly. We were missing .md class so some markdown styles were not applied

Testing

1. Go to a group iteration (Issues > Iterations in left sidebar)
2. Create an iteration cadence and save it
3. From the list page, expand the Cadence then click on one of the iterations
4. Ellipsis menu => Edit
5. Add some markdown to the description (in particular a table)
6. Confirm it renders correctly

before	after