[pgAdmin4][Patch]: RM#1261 - Fix corners and width of backgrid SQL fields.

Please find attached patch for issues:

1) RM#1261

The border was applied to outer div instead it should be applied to the CodeMirror class.

so It seemed to get more width. 

Also applied z-index CodeMirror class to fix breaking corner issue.

2) In properties panel while scrolling the page, the content of page seems to move over the navigation bar with edit button. Please see attached screenshot.

It was the issue of z-index which is fixed. 

Please review.

Thanks,

Surinder Kumar