* Added support for taking screenshot
- Screenshot is taken using html2canvas package
- Converted to base 64 and uploaded to metadata
- For Using first display output
- Notebok object is passed instead of string, to publish pane
- The first cell with output present is parsed out
- The dom is also parsed to get corresponding div element to take screenshot of the first output
* fixed bug
* Addressed PR comments
- FIxed bug that didn't capture screenshot when mutiple notebook tabs are opened
* removed unnecessary dependencies
* fixed compile issues
* more edits
* Added support for custom image upload
- Dropdown gives an option for URL or image upload
- Preview shows how the card will be displayed in the gallery
- base64 converted image stored in metadata document
- Max limit is 1.5MiB for the image
* fixed lint errors
* addressed PR comments
- Added test
* added snapshot
* fixed failing test
