This PR makes the `markdown_preview_font_size` setting be based on the
UI font size instead of the buffer UI font size. I typically use a much
smaller code font than UI font, and that made reading the markdown
preview super small. I don't think this will be uncommon because the
dimensions between mono and non-mono typefaces are different...

Release Notes:

- Changed the markdown preview to fall back to the UI font size instead
of the buffer font size when `markdown_preview_font_size` is unset.