Jupyter-flex

- Refactor JS modules and release to NPM
- Multiple small CSS and JS fixes

- Added back the missing flex_logo and flex_favicon options

- Fix include_source behaviour to be the same in Voila and nbconvert

- Moved all Frontend code from the gigantic monster Jinja to React!
- Replaced `chart` and `text` tags with `body`, tagss reflects better location instead of type
- Added option to have "scroll" vertical layout, default is the same "fill"
- Added option to have help button on the card header that shows a modal with cell content
- Added option to have individual Page sidebars, Global Sidebar is still possible
- Added better rendering for Sidebars and markdown on body cells
- Improved rendering of widgets to be one at a time when needed instead of all at once
- Improved and standardized rendering of multiple data-types
- Improved Sectioin and Card layout limits when content was bigger than the space available
- Added option to show the card source cells using `flex_include_source` (default es Voila `strip_source`)
- Renamed `flex_source_code` to `flex_source_link`, reflects better what it does
- Fix issue with qgrid by including qgrid.js

- Added the option to have a global sidebar
- Remove the explict option of section sidebar, still possible useing size tags
- Improved ipywidgets default styling
- Added history tabs for the main navigation
- Added material colors CSS classes for easy customization
- Support for qgrid widget

- Standardized all parameters with a flex\_ prefix
- Added flex_subtitle, flex_logo, flex_favicon parameters
- Added option to include a custom.css file using flex_custom_css
- Added option to have multiple outputs per card, including markdown cells
- Added option to add custom CSS classes using `class={value}` tags
- Changed default theme