Use bootstrap-like columns for the container macro (columns layout)

Currently, the columns layout of the container macro is doing a columns layout that is not based on bootstrap.
Since XWiki 6.4.x, bootstrap is available in XWiki and it would be interesting to make that column layout with bootstrap grid / columns, one of the benefits being responsiveness.
Responsiveness was already implemented in XWIKI-10135, but without using Bootstrap columns (see also XWIKI-14210).

However, for reasons explained in the comments of XWIKI-14210, it's not really possible to use the bootstrap grid for this columns layout, but we can use a very similar strategy, for coherence purposes. Coherence would also be visual, as the margins & paddings will look the same as the bootstrap ones.