Resize gadget thumbnail image in add gadget panel based on customer feedback

Issue Summary

The new add gadget panel shows the gadget thumbnail in 1:1 aspect ratio, which is different from the 2:1 ratio in the old add gadget modal. As a result,  thumbnail images are cropped as squares with a full height but a reduced width. As a result all thumbnails look broken. 

Steps to Reproduce

1. Go to Jira dashboard 
2. Click on add gadget 

Expected Results

Expect to see uncropped thumbnail 

Actual Results

Thumbnails are cropped and incomplete 

Workaround

No workaround required, the visual bug will be fixed by 9th Sep