DashboardGrid: Change grid margin to 8, to align to 8px grid (grafana#21167)

Author: torkelo

public/app/core/constants.ts

@@ -1,5 +1,5 @@
 export const GRID_CELL_HEIGHT = 30;
-export const GRID_CELL_VMARGIN = 10;
+export const GRID_CELL_VMARGIN = 8;
 export const GRID_COLUMN_COUNT = 24;
 export const REPEAT_DIR_VERTICAL = 'v';