Improve Snackbar and Fab button to make app more agnostic from Flagship application

[cballevre]

One of the problems encountered in applications with a snackbar or fab button is managing the margin at the bottom if the flagship application is in immersive mode. Example into Home :

• https://github.com/cozy/cozy-home/blob/8761d6da5e834042b2e935f1ae6ab5ad90afaa3b/src/components/AddButton/AddButton.jsx#L27

Perhaps we could manage this logic on the cozy-ui side rather than in the apps themselves