Data pages: switch to a new markdown parser and renderer

[danyx23]

Our current markdown parsing and rendering logic was handrolled because we had very limited syntax needs for chart subtitles and footnotes specifically and wanted to support custom DoD syntax.

Our needs have changed now and with authoring many more metadatafields in the ETL as markdown our desire to support more markdown features has grown. We could extend our current custom parser but we heard about some advanced requests like parsing/rendering formulas that we wouldn't want to handroll.

Requirements

• Separate parsing and rendering steps (since we will need to render chart subtitles ourselves in svg text with support for measuring svg text width)
• Ability to process the markdown AST (so that we can rewrite items that make sense in a certain context like headings)
• Should come with a capable renderer that integrates with react 16
• Support for syntax extensions (e.g. superscript/subscript, ideally also advanced stuff like formulas)

Implementation plan

Must have

• Research existing libraries
  • https://github.com/remarkjs/react-markdown
  • https://github.com/markdown-it/markdown-it
  • Maybe others?
• Proof of concept that plugging it into our subtitles rendering works
• Use the new component for rendering markdown in on the main panel of Grapher
• Use it to render the sources/about tab fields
• Use it to render markdown ETL fields on data pages

Nice to have

• Superscript/subscript
• <br> tags
• Tables
• Formulas
• Consider adding a markdown component to gdocs authoring (especially useful if we were to add tables and/or formulas as we don't have a good solution for those in GDocs either)