[IMP] web_editor, *: change height/margin system to padding system

* web, website, mass_mailing

Before this commit, the editor allowed defining the margin of some
elements and the height of some elements. There were three main
problems with this system:

1) When an user dragged the arrow, he often expected to change the
   size of the element while he was in fact changing the margin

2) When changing the margin, you increase the space between two
   elements but with a white space (while most users want a space
   with the same color of the block they are resizing = padding)

3) When changing the height, the user was able to hide snippet parts
   and the height was not consistent accross screen sizes

Now, the only thing you can edit is padding. There is only one arrow
for each direction, which allows increasing spacing and consistent
dimension at the same time.
The only feature that is lost with this change is the ability to
control vertical alignement of colored columns. This is not a problem
as colored columns are rare in our current snippets and there still
have the possibility to add margin classes by hand if necessary (there
are also plans to control vertical alignement another way).

Note: margin classes are kept for compatibility.