Add shadow to sticky footer

During the user testing, some users experience difficulties finding the new sticky footer component. The general consensus is that the component will be more obvious by applying a shadow to the element.

the proposed shadow is the standard bootstrap shadow color and length (length=1rem color=rgba($black, .15)).

However, we cannot use the "shadow" class because the bootstrap shadow is not centered on the element but a bit below. The solution is to add this rule directly into the .stickyfooter SCSS:

box-shadow: 0 0 1rem rgba($black, .15);