[pgAdmin4][Patch]: SqlFieldControl enhancements

Hi

Following are the enhancements:

1) Disable cursor mode if editor is disabled.

if cursor is enabled. on pressing keys, a false event is generated and on validating models it causes Save button to enable.

2) If editor is disabled, set dark background colour.

3) CodeMirror corners seems to cut around. CodeMirror z-index is greater than pgadmin-control class which causing issue. it is fixed.

4) CodeMirror not taking full height. set line-height css property to fix the issue.

Please find attached patch and review.

Thanks,

Surinder Kumar