Facia re-naming: collection becomes slice

Follows the new naming conventions:

#### Slice:

> The row of  items within a container. A container is made up of 1 or more slices. A slice can contain 1 or more items.



Note:  Renaming one concept per branch into `facia-naming-global` branch, which will be merged later all together (as soon as we merge the new template logic for the fronts).

![ ](http://media.giphy.com/media/GLXpLpnXLchu8/giphy.gif)