Render markdown in tooltip with post-utils, instead of ReactMarkdown

[mickmister]

The webapp has a robust markdown system that I think we should utilize instead of using the ReactMarkdown library:

mattermost-plugin-github/webapp/src/components/link_tooltip/link_tooltip.jsx

Lines 118 to 122 in 03935ba

	<ReactMarkdown
	source={data.body}
	disallowedTypes={['heading']}
	linkTarget='_blank'
	/>

The todo plugin uses the webapp's exposed post-utils to render its markdown. I think it makes sense to do the same here. This would allow for feature-rich things like at-mentions to be rendered as well, assuming the backend does the same sort of translations of GitHub -> Mattermost usernames before the PR description is delivered to the frontend, as it does when it creates posts from a GitHub event.