This commit adds the possibility to use a grid to move and resize
columns inside some building blocks.

More precisely, for the majority of the building blocks having columns,
it is now possible to toggle between the current layout (= normal mode)
and the grid layout (= grid mode). The grid mode can be triggered in
two ways:
	- by using the "Grid" button in the options (when available),
	- by dragging a column using the move handle.

The toggle places the different columns in the grid as close as
possible as how they were placed in normal mode.

The normal mode is toggled back by clicking on the "Cols" button (when
available).
Since the move handle is now used to trigger the grid mode, the columns
in normal mode are now moved using the arrows.

NB: to avoid any confusion with the word 'column' in the explanation
below, the BS column will be called 'grid item' and the grid column
will be 'column'.

When in grid mode, the building block is a grid with 12 columns and a
number of rows which depends on the height of the grid items.
This mode offers the following functionalities:

- Moving a grid item:
When a grid item is dragged, it can be placed anywhere in the grid. If
it is dragged towards the bottom, new rows are added in the grid to
increase its height and to place the grid item lower. These added rows
are removed when it is dragged towards the top.

  A grid item can also be placed over an other item (-> they can overlap)
  and their order can be changed using the two new buttons to place an
  item in front of/behind all the others.
  
  A grid item can be moved from a grid to another, and also from a grid
  to a building block in normal mode.

- Resizing a grid item:
A grid item can be resized vertically, horizontally and diagonally with
the help of the grid. When resized towards the bottom, new rows are
added in the grid and they are removed if it is towards the top (like
with the drag and drop).

- Adding elements:
The "Add Elements" option allows the user to add as many images, text
blocks or buttons as wanted in the grid.

  Inner contents can still be dragged and dropped inside a grid item.

- Changing the padding of the grid items:
The padding option allows to change the vertical/horizontal padding of
all the grid items at the same time.

- Mobile view:
When in mobile view, the grid items are not in a grid anymore (the
display is back to flex) but they keep their grid properties. They
cannot be resized, nor dragged and dropped but they can be moved using
the arrows. By moving this way, it does not impact the desktop view.

task-2825241