Make react-markdown optional

[elboletaire]

Right now react-markdown isn't optional. It needs to be installed as a different package in order to get a smaller bundle size.

What we should do is dynamically import react-markdown (and remark-gfm, a required plugin), only when markdown parsing is enabled (via some context setting). Doing it this way, the required react-markdown and remark-gfm packages will be only required when actually parsing markdown contents.