Replace Edit-Preview modes with in-place WYSIWYG

[cvharris]

Instead of having one view of the card with raw text and the other view having the katex-ified preview, it may be desirable to streamline the UX by not having to toggle between modes to see your progress.

At first this app had two cards, an editable one on top and one with the preview in readonly view below. However this was difficult to fit on the screen just right.

An in-place WYSIWYG is very difficult to create and likely requires a custom parser. Issues that would have to be solved:

• Maintaining cursor position as text updates from raw to formatted
• Handle keyboard movement across visible characters only
• Handle delete/backspace on certain invisible characters
• Maintain both raw and formatted texts separately in state