#607 Convert worksheet list from Angular to React

- Rewrote worksheets/index.html to remove all Angular references
- Added worksheet_list.js to list worksheets with React
 - Worksheets in list have concept of 'focus', similar to worksheet items
 - Added keyboard shortcuts to navigation worksheet list: j/k up/down, and x or Enter will navigate to focused worksheet
- Slightly rewrote markup for worksheets in list
- Slightly restyled worksheets in list
- Added up/down keyboard shortcuts to worksheet details also