Use markdown and update chart visibility and order

[tildechris]

Convert to use markdown instead of big HTML chunks in app.R.

In addition, always show the score description and incident chart; move the footnote text to the bottom of the screen.

[tildechris]

@kateharwood let's chat about the layout tomorrow. I see where you were indenting the content but the content areas were already aligned.

It seems you wanted the text to align with where the y-axis on the plots start, which we can do, but that's semantically a bit different.

This shows the explanation indented one "column" and the "Some forecasters" aligned with where the figures start.

[kateharwood]

I agree the 1 col looks fine (but I do think all the text should be aligned at the same indentation). This PR changes the functionality of the score explanations, did you intend that?
Before (score explanation is in dropdown, not automatically shown):

After:

There is so much text that is under the main plot (and I am about to add more per Nick's request) explaining methods / giving disclaimers that I'm not sure we want more showing by default. Or at least we should have it separated or with a "Explanation of Score" header if not the dropdown.

The score explanation also flashes at the top before the app finishes loading the plots.

[tildechris]

[tildechris]

@kateharwood this is ready for review now.

[kateharwood]

I still think it's really odd that the description shows up at the top before the plots load. It's more than a negligible amount of time. I wonder if there is a way around that. I also feel like the score explanation text should stretch the width of the plots rather that cutting off part way.