Render video descriptions as markdown

Fixes #23 (closed)

The video descriptions are now rendered as markdown. The renderer uses a similar but simplified configuration to the one used in the peertube client.

Current

This MR

Notice that in order to make it easier to distinguish the video description from the rest of the UI, I added a left border to it. If you feel like it's not any good, I'll remove it.

Without the left border