Add markdown (and mathjax) support in leaderboard descriptions

[skimberk]

As per #38 this adds Markdown support in leaderboard descriptions. With the v2 frontend, this looks a bit different than the attempt in #39. It also looks like the v2 frontend lost support for mathjax in the descriptions (although I'm not sure if it's actually used anywhere?) so this adds it back, as well as adds mathjax support to the news page (since they both now use the same MarkdownRenderer component.

Mostly followed instructions here for getting mathjax working with react-markdown: https://github.com/remarkjs/react-markdown#use-remark-and-rehype-plugins-math

One issue:

• A lot of leaderboard descriptions currently use a single newline for line breaks, whereas Markdown collapses these, and needs two newlines to actually make a line break. I guess the descriptions can just be modified, there aren't that many as far as I can tell/doesn't seem like too big an issue anyway.

Ran tests locally:

npm run test

Also tried adding some markdown and math to a description to test how it renders (see $$E=mc^2$$ at the bottom):

I actually wrote this one by hand, as I can't say my trust in CoPilot is especially high after dealing with it in #97, I feel like I spent a long time just trying to figure out what the hell it was doing.

@b9r5 @msaroufim