Replace react-markdown with react-remark

[DingDongSoLong4]

In the help menu, there is a noticeable lag every time a new tab is opened. This replaces the react-markdown plugin with react-remark, which is significantly faster and basically removes this lag entirely. I did try updating react-markdown to the latest version first (8.0.3, currently on 7.1.0) but the lag was still there.

This should also improve performance in all the other places where MarkdownPage is used.

Edit: okay well this was working when in the debug server but now it isn't in a release build, there's probably something weird going on with the dependencies.