Replace loading icon GIF with pure CSS loading icon

The pure CSS loading icon uses an ::after pseudo-element, and thus it
can only be used on container elements; the input element is not a
container element, so when the class "icon-loading-small" is set on an
input element it falls back to a loading icon GIF.

This commit modifies the markup of the login button to add a div element
that holds the confirm icon and the loading icon (depending on the
case). In order to position the icon in the same place as before the
input and the icon are both wrapped by another div making possible to
set the absolute position of the icon relative to the input.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>