[PRO] Change .input-group-addon background when focused

[jchernan]

Paper Dashboard v1.1.0 changes the background of an .input-group-addon by adding class .input-group-focus. This is missing in Paper Kit:

https://github.com/creativetimofficial/paper-kit/blob/master/assets/sass/ct-paper/_inputs.scss

Code from Paper Dashboard v1.1.0:

.input-group-focus .input-group-addon{
    background-color: $white-bg;
}