[TASK] Remove unneeded padding in page module

Removes excessive top margin above Web>Page backend layout
cells. Brings gridCells equal heights and bottom spacing. Replaces
full white borders with adequate margins.

Change-Id: Ic5e7d3279c4facd969f171280d881b5620df4be7
Resolves: #55150
Related: #39971
Releases: 6.2
Reviewed-on: https://review.typo3.org/26936
Reviewed-by: Benjamin Mack
Tested-by: Benjamin Mack
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters