Bug #1822911 User icons not appearing circular in some themes

Use object-fit:cover to force non-square images to size to the min
width/height of the image loaded so that it displays the center of
the image in the correct ratio, hence fixing the issue of oval shapes.

Added extra classes to tell theme what size the images are to display,
ie .user-icon-20, .user-icon-40, .user-icon-60, etc, and
.user-icon-inline for when username is displayed inline with user icon.

behatnotneeded

Change-Id: I28e0b37d0961c3be0195f4baef704262a0547af0