Enable panel size to be defined in dashboard yml
This is a part of the original scope of #27834
Problem to solve
Currently we allow users to define a dashboard, but the panels are always arranged in a side by side layout on the screen and on one line on mobiles
However, users would like to be able to more easily resize items.
- We should accommodate to different screen sizes
- We should define which units (%, # cols, fixed widths) to use
#27834 attempted to address this issue, but its scope was too large to be manageable. As part of the effort to reduce the scope we created this issue.